default-apps: Workaround AdwComboRow item selection at startup
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 6fb32463
Fixes #2719 (closed)