PBS authored Apr 18, 2024 and Marc Jeanmougin committed May 08, 2024

* Clean up src/extension headers.

* Merge copypasta'd CDR and VSD import dialogs into RvngImportDialog.
  Ensure RvngImportDialog destroys the document after the viewer.

* Fix sp_gradient_load_from_svg() to return the cloned object rather
  than the original, like sp_marker_load_from_svg().

* Convert various cancelled flags to return by std::pair, and change
  error checking to not print messages on user cancellation.

* Fix crash on PaintServersDialog/SymbolsDialog destruction from missing
  invoke_hide().

* Fix leaks in SymbolsDialog by splitting SymbolSet into an owning
  variant (SymbolSet) and a non-owning one (SymbolSetView).

* Replace ea92f8e8 with a sigc::mem_fun-based solution, as the former
  might not work if the Canvas is Gtk::managed (it's not at the moment).

* Cope with the Canvas being destructed without being unrealised. This
  can now happen in GTK4 if the C++ wrapper is deleted while something
  holds a reference to the C widge...