[Gtk4Prep] BreadcrumbEntry: Use EventControllerKey

[jeremypw]

Behaviours that should be unaffected when the entry is focused are

• Tab completion (Tab)
• Change entry to parent path (<Control>Up)
• Exit edit mode and focus view (<Control>Down)
• Do not exit if Enter pressed in search mode

Cleanups

• Merge single-use or one-line functions
• Do not use event.is_modifier
• Do not try to handle shortcut already used as action accelerator by Window

[danirabbit]

Appreciate the notes for things to test!