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/Widgets/WebViewHostDialog.cpp

@@ -75,6 +75,8 @@ if(document.documentElement)
 
 } // namespace
 
+std::string WebViewHostDialog::theme_apply_script() { return host_theme_apply_js(); }
+
 // Document-start user script: injects the contract <style>, stamps data-orca-theme before
 // first paint, and raises a JS flag so the legacy globalapi.js dark.css poll stands down for
 // host-themed pages. The WebView2 timing guard lives in document_start_injector().