Our callback handler for the AdwComboRow::selected-item property
changes gets called at startup, resulting in a slow and unnecessary
default app change.
This is a workaround to avoid calling g_app_info_set_as_default_for_type
without user input.
This is a backport of commit 6fb324634aFixes#2626Fixes#2683
GTK has deprecated the AppChooser interface and related widgets.
These APIs will be gone in GTK5, so we should move away from
them as soon as possible.
This MR takes the opportunity to change the design to match newer
mockups while dropping deprecated APIs.