Restore Plugin HTML After a WebKit Reload (#15737)

Plugin dialog content is injected with SetPage, so on the WebKit backends a
reload (context menu, keyboard shortcut or location.reload()) re-fetches the
SetPage base URL instead of the injected document, and the plugin UI is gone
for good: load_plugin_content() returned early once m_content_loaded was set.

Re-inject the plugin HTML when a main-frame load after the initial swap is
neither that swap nor a page the plugin linked to. m_own_page_load marks the
load our own SetPage caused, and the URL test recognises the reload: the
injected document and the directory a reload re-fetches both report the base
URL, so a load of any other URL is left alone. The test ignores a fragment the
page navigated to, and undoes the escaping the web view applies to what the
resources path holds.

A load reaching the base URL is not enough on its own, because WebKitGTK reports
a navigation that never committed against the document that stayed and then
finishes that document again: a link to a missing file therefore arrives as a
load of the base URL and reads exactly like a reload. So the re-injection also
requires a navigation to the base URL to have committed, which a reload always
does and a failure never does.

A bootstrap page that cannot be loaded is still not recovered from: WebKitGTK
substitutes a stock error page for it, and that load supersedes the swap
whichever way the swap is ordered around it. The file ships, so this is a
broken-install path; nothing here makes it worse than it already was.

No separate MSW path is needed: wxWebViewEdge ignores the SetPage base URL, so
its documents report about:blank and the test never matches there, and WebView2
reloads NavigateToString content from its own history entry anyway.

src/slic3r/GUI/PluginWebDialog.hpp

@@ -64,6 +64,7 @@ protected:
 
 private:
     void on_bootstrap_event(wxWebViewEvent& event);
+    void on_navigated(wxWebViewEvent& event);
     void load_plugin_content();
     void on_close_window(wxCloseEvent& event);
     void fire_submit(const nlohmann::json& data);
@@ -72,6 +73,8 @@ private:
 
     std::string                   m_html;
     bool                          m_content_loaded{false};
+    bool                          m_own_page_load{false};   // a SetPage of the plugin HTML is in flight
+    bool                          m_content_navigated{false}; // a navigation to the base URL has committed
     bool                          m_open{true};
     bool                          m_close_fired{false};
     std::optional<nlohmann::json> m_result;