Modernized preferences

This updates the preferences UX, specifically:

• Switches the preferences dialog(s) to AdwPreferencesWindow and aligns better with the GNOME HIG
• Moves game-specific settings into a separate "new game" dialog, shown when starting a new game

This MR includes a slight modification of the settings binding, that is now hidden behind a more strongly typed abstraction. The utility of this is somewhat underwhelming, as I haven't refactored the rest of the code base to benefit from this, so at this point it mostly just helps keep the actual dialogs clean from the settings update logic. As a side effect of all of this, all settings are now bidirectionally bound as well, so e.g. updating settings from somewhere else (if that should ever become relevant) should just work.

Note that the new UX and aligning texts with the HIG creates a bit of additional translation work.

Closes #56 (closed)