preferences-dialog: Port to libadwaita widgetry

[jannuary]

Ports the preferences-dialog to libadwaita's Preferences* widgets, putting it in line with the common GNOME preferences pattern.

Notes:

• This requires a prerelease version of libadwaita due to EntryRow, so it cant be released on Flathub before GNOME 43. (at least without bundling libadwaita)
• This saves settings as the window closes. This isn't ideal - the standard is instant apply.
• ~~Plugins tab hasn't been ported over yet.~~
• Plugins are now trashed instead of being deleted. I need to figure out how to show a notification allowing to untrash it, as well as handle possible errors.
• This ignores CSD being disabled. Unsure how supported that case is with PreferencesWindow anyway.
• There's a lot of warnings about mismatched types.
• Last but not least, I'd like to revisit and reorganise the settings design-wise. A lot of these seem either poorly explained or simply not needed.