Add a Dockable HTML Panel API for Plugins

orca.host.ui.create_dock_panel(html, title, width, height, on_message,
on_close, dock) hosts plugin HTML in a pane of the Plater's dock manager,
next to the sidebar, and returns a UiDockPanel handle
(post/show/hide/close/is_open). The arguments follow create_window(). The
panel uses the window.orca bridge of plugin windows, restores its position
and size from the saved window layout, hides with the Plater off the Prepare
and Preview tabs when floating, and is closed with its plugin; plugin panes
are removed in MainFrame::shutdown().

The web view hosting moves out of PluginPage into a shared PluginWebPanel
base: bootstrap page and swap to the plugin HTML, theme, element-default and
bridge scripts, window.orca message parsing, delivery to the page, and live
re-theming, also re-applied on every load after the swap. Pages tabs and
docked panels both derive from it. Pages tabs now re-theme in place on a
theme change instead of being reloaded, and a window.orca call a host does
not support is logged.

What the hosts share no longer lives in one of them: the bootstrap page, the
base URL and the plugin-window bridge move to Widgets/PluginWebContent, used
by PluginWebDialog and PluginWebPanel alike, and the saved-layout parser
moves to Widgets/AuiPaneLayout, so the Plater does not depend on a plugin
widget.

Includes a sample plugin (sandboxes/orca_dock_panel_plugin_any.py) and
binding and layout-helper tests in slic3rutils.

src/slic3r/GUI/PluginWebDialog.cpp

@@ -1,11 +1,7 @@
 #include "PluginWebDialog.hpp"
 
-#include "slic3r/GUI/GUI.hpp"
 #include "slic3r/GUI/GUI_App.hpp"
-
-#include <libslic3r/Utils.hpp>
-
-#include <boost/filesystem.hpp>
+#include "slic3r/GUI/Widgets/PluginWebContent.hpp"
 
 #include <wx/event.h>
 
@@ -13,50 +9,6 @@
 
 namespace Slic3r { namespace GUI {
 
-namespace {
-
-// Injected into the top-level page at document start (before the plugin's own
-// scripts). Defines window.orca as the only host surface the page may use. It
-// references window.wx lazily (at call time) so it never races the backend's
-// deferred registration of the "wx" message handler. Guarded against
-// double-injection so it is harmless if also prepended.
-constexpr char ORCA_BRIDGE_JS[] = R"JS(
-(function () {
-  if (window.top !== window.self) return;
-  if (window.orca) return;
-  var handlers = [];
-  function send(kind, data) {
-    try {
-      window.wx.postMessage(JSON.stringify({
-        channel: 'orca', kind: kind, data: (data === undefined ? null : data)
-      }));
-    } catch (e) { /* bridge not ready yet */ }
-  }
-  window.orca = {
-    postMessage: function (d) { send('message', d); },
-    submit:      function (d) { send('submit', d); },
-    close:       function ()  { send('close'); },
-    onMessage:   function (cb) { if (typeof cb === 'function') handlers.push(cb); }
-  };
-  window.__orcaDispatch = function (payload) {
-    var data = payload ? payload.data : null;
-    for (var i = 0; i < handlers.length; i++) {
-      try { handlers[i](data); } catch (e) {}
-    }
-  };
-})();
-)JS";
-
-// file:// base URL for plugin HTML loaded via SetPage, so self-referencing
-// relative URLs resolve against the bundled web resources directory.
-wxString web_base_url()
-{
-    const std::string dir = (boost::filesystem::path(resources_dir()) / "web").make_preferred().string();
-    return wxString("file://") + from_u8(dir) + "/";
-}
-
-} // namespace
-
 PluginWebDialog::PluginWebDialog(wxWindow*          parent,
                                  const wxString&    title,
                                  const std::string& html,
@@ -75,7 +27,7 @@ PluginWebDialog::PluginWebDialog(wxWindow*          parent,
 {
     // A tiny bundled bootstrap page brings the webview up; the real plugin HTML
     // is swapped in via SetPage once the bootstrap finishes loading.
-    create_webview("web/dialog/PluginWebDialog/blank.html", title, size, wxSize(320, 240));
+    create_webview(plugin_web::BOOTSTRAP_PAGE, title, size, wxSize(320, 240));
 
     // Paint the window/webview in the themed background so there is no white
     // flash before the (transparent) bootstrap page and plugin HTML render.
@@ -97,7 +49,7 @@ void PluginWebDialog::add_user_scripts()
 {
     if (wxWebView* wv = browser()) {
         wv->AddUserScript(wxString::FromUTF8(WebViewHostDialog::plugin_defaults_user_script()));
-        wv->AddUserScript(ORCA_BRIDGE_JS);
+        wv->AddUserScript(wxString::FromUTF8(plugin_web::orca_bridge_script()));
     }
 }
 
@@ -151,7 +103,7 @@ void PluginWebDialog::load_plugin_content()
         return;
     m_content_loaded = true;
     if (wxWebView* wv = browser())
-        wv->SetPage(wxString::FromUTF8(m_html), web_base_url());
+        wv->SetPage(wxString::FromUTF8(m_html), plugin_web::content_base_url());
 }
 
 void PluginWebDialog::on_script_message(const nlohmann::json& payload)