base:navigation: Cleanup navigation key modifiers enum

base:navigation: Cleanup navigation key modifiers enum

We were exposing the 'ALT' modifier as if we were guaranteeing its
accuracy but truth is we were only exposing configuration dependent
values.

Make the API simpler for now, the same way as Gtk3 was exposing it, and
when we have time to guarantee more values by making them take backends
configuration into account, we will expose those values in a accurate
way.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1402

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3565>