Pinta 2.0 - now ported to GTK 3 and .NET 6

This is a major new release of Pinta, which has now been ported to GTK 3 and .NET 6:

- Many changes to the appearance of standard GTK widgets and dialogs (e.g. the color picker and file dialogs). GTK3 themes should also now be supported.
- Improved support for high-DPI displays.
- The platform-native file dialogs are now used (#1909807, #1909664).
- A GTK version of 3.24.21 or higher is recommended. Earlier versions have a bug with changing the file filter in the Save As dialog (#1909807).
- On macOS, the menu now appears in the global menu bar instead of the application window.
- Changed the text tool to use the standard GTK font chooser widget (#1311873, #1866653, #890589)
- Changed several tools to use spin buttons rather than editable combo boxes for e.g. selecting brush sizes (#1186516).
- The Open Recent menu item was deprecated in GTK3 and has been removed, but similar functionality is available in the file dialog’s Recent section.
- Support for add-ins has been removed, but may return in a future release (#1918039).
- The Windows and macOS installers now bundle all necessary dependencies. Separately installing GTK and .NET / Mono is no longer required.

- Added a status bar widget containing the position / selection information, zoom, and the color palette (#154)
- Changed the tool palette to be a single column (#155)
- Added recently used colors to the color palette widget (#154)
- Tools now save their settings for the next time Pinta is opened (#178).
- The primary and secondary palette colors are now saved in the application settings (#171).
- The canvas can now be panned by clicking and dragging with the middle mouse button (#176, #419).
- On macOS, keyboard shortcuts now use Command instead of Ctrl.
- The macOS installers are now signed and notarized.

- The Paste Into New Image action no longer creates several unnecessary history items (#170).
- Performance improvements for the paint bucket and magic wand tools (#159).
- Performance improvements for the selection tools when interactively adjusting the selection.
- Removed the Images pad, which is obsolete now that tabs are used (#153).