Common components that are used in multiple panels

These changes involve retiring custom widgets in favour of libadwaita widgets.

Initiatives#51

These changes need a release freeze break request. See https://handbook.gnome.org/release-planning/freezes.html

Default Apps panel

This merge-request needs a maintainer or someone with domain-specific knowledge to review.

This merge-request has been reviewed and the author is expected to address the review before the changes are merged.

Apps panel (includes Default Apps and Removable Media settings)

Issues which cause the openQA end-to-end tests of GNOME OS to fail in 'master'.

Initiatives#44

Initiatives#43

Initiatives#42

Human-perceptible delays in response time to user actions, or issues related to the excessive use of system resources like CPU, I/O, GPU, etc.

This merge request has been accepted but should only be merged once the module emerges from code freeze (see https://wiki.gnome.org/ReleasePlanning/Freezes)

Issues related to the "System" panel

Refactoring, technological debt, etc.