Add SettingsPopover Class

[Marukesu]

move the settings menu popover, and related functions to they own class.

the settings object is now, also used to keep the state between the widgets synchronized, and the actions provided by it make the new class mostly self-contained, only needing a single signal to indicate the window that the theme dialog should be shown.

[jeremypw]

@Marukesu This seems like a good idea especially as MainWindow.vala is larger than I would like. Could you fix the conflicts please?

[Marukesu]

@jeremypw, sorry for the delay in rebasing this, should be good to review.

don't know what caused the CI error after the rebase, but with the --print-errorlogs option, it should be more easy to know what test case failed in the future.

[jeremypw]

This gives two new compile warnings with valac 0.56.7

../src/Widgets/SettingsPopover.vala:180.13-180.32: warning: Use of possibly unassigned parameter `css_provider'
  180 |         if (css_provider == null) {
      |             ^~~~~~~~~~~~~~~~~~~~   
../src/Widgets/SettingsPopover.vala:186.9-186.90: warning: Use of possibly unassigned parameter `css_provider'
  186 |         style_context.add_provider (css_provider, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);

Do you know if later versions of Vala will support it? I tend to avoid this pattern though it is useful.

[jeremypw]

Despite the coding technicalities there do not seem to be any regressions in operation.

[jeremypw]

Found a regression: Changes in font size are not restored after closing and reopening the app.

[jeremypw]

@Marukesu Another regression with this is that the font scale is not set individually on each tab.

[Marukesu]

I'm tempted to revert the ActionGroup change for now, the widget won't be as self-contained as i would like, but would still make this a good start in reducing the MainWindow scope.

[jeremypw]

Yes two smaller PRs would be easier to review.

[jeremypw]

Code looks good, no regressions or new terminal warnings seen.