feat: add default language setting

[mxmgorin]

Description:
Adds a 'Default layout' option that makes the keyboard always start in a chosen language when a new text field is opened, instead of keeping the last used. Defaults to "Last used" to keep current behaviour.

Reason:
Currently, after switching language, it stays until you manually switch back. If you type mostly in one language, this means constantly changing the layout. Pinning a default language makes the keyboard reliably return to primary language on each typing and you always know which language is currently selected.

[pzauner]

Great addition, will review this soon!

[pzauner]

Merge has some issues:
The new preference key is missing from the backup/restore schema.
When the selected layout is deleted, renamed, or hidden, an invalid selection remains stored.
The change is tied to onStartInputView; when using a hardware keyboard, onStartInput is the more reliable lifecycle hook.
Registering additional subtypes and the immediate switch occur directly one after the other. The Android subtype list may still contain the old state during this process.

I can fix them myself though.