Port to GTK4

[Antolius]

Port from GTK3 to GTK4. Underlying elementary stack (library support, Granite, stylesheet, etc.) is a blocker for completing this one, but some preliminary work can be done in advance.

[Antolius]

Dependency on gtkspell3 introduced in input page improvements will make this difficult. Will need to find some alternative to this library, potentially implement the UI part in Starfish itself.

[Antolius]

Dependency on gtkspell3 introduced in input page improvements will make this difficult. Will need to find some alternative to this library, potentially implement the UI part in Starfish itself.

Ok, looks like GtkSpell has been superseded by gspell, which is actively maintained, and even has a (for now unofficial?) gtk4 port. So that's good.

[Antolius]

Alternative might also be libspelling.