Add window with catalog of keyboard shortcuts

[robertoffmoura]

This addresses issue 1434 by adding a dialog with all keyboard shortcuts and their actions. This dialog can be accessed via Pinta > Settings or via keyboard shortcut with ctrl + comma. The shortcuts are separated by category, such as tool shortcuts, layer shortcuts, and top bar menu items. The shortcuts are searchable by clicking the magnifying glass icon on top of the dialog.

[badcel]

Did you see that there is a special adwaita shortcut window?

https://gnome.pages.gitlab.gnome.org/libadwaita/doc/1-latest/class.ShortcutsDialog.html

[robertoffmoura]

ShortcutsDialog was introduced in Adw 1.8, but it looks like Pinta requires libadwaita 1.7+. Would it be OK to update the minimum version requirement of Adw to 1.8?

[JGCarroll]

Libadwaita 1.8 would prevent making .deb packages for Debian 13, which isn't done officially anyway, but could knock on any third party repos that might wish to do so.

If we were to update Libadwaita, we might as well update Libgtk alongside it; if Debian 13 stops being supportable directly, it'd make sense to pin the requirements to match Ubuntu 26.04 instead which has Libadwaita 1.9 and GTK 4.22, other distributions are generally rolling release already or have a much faster release cycles in general (e.g., technically Pinta wouldn't support Fedora 43 either, but Flatpak exists and 43 will be EOL by November; Debian 13 is likely the only "real" loss).

This assumes that Flatpak/Snap runtimes are updated with those libraries, Snap isn't quite ready yet, Flatpak I don't know directly but expect is probably fine, by the time Pinta 3.2 releases I wouldn't expect either to be lacking support.

[cameronwhite]

I'd be fine with bumping the required version as long as snap supports or will soon support it - snap & flatpak compatibility is the main mandatory requirement, although being able to build on Ubuntu and other recent distros is a nice bonus.

The other option here would be using https://docs.gtk.org/gtk4/class.ShortcutsWindow.html which would work as-is (I assume its deprecation is because the Adw.ShortcutsDialog will replace it)