Alice Mikhaylenko authored Aug 09, 2021 and Adrien Plazas committed Aug 12, 2021

When using a layout manager, get_request_mode() comes for free, but
otherwise it has to be implemented manually for every widget.

Coincidentally, we've had a copy of GtkLayoutManager's get_request_mode()
in AdwHeaderBar and AdwViewStack. Extract it into widget utils and use it
everywhere.

Fixes #70