appchooser: Improve categories style and make them checkable
appchooser: Category selection

Change category when selected.
appchooser: Exclusive group for mode switcher
appchooser: Hide mode switcher for now

Grid layout needs improvements so it's not ready.
appchooser: Search results

Make search work.
appchooser: Never show description if empty or undefined

This also fixes warning when trying to set text with an
undefined value.
appchooser: Properly align runner results header
appchooser: Use heading for runner results
appchooser: Don't set snap mode for runner results
appchooser: Relayout results list when runner model changes

Force a relayout.
appchooser: Enable only some runners
appchooser: Add ProcessRunner::runUserSettings()
appchooser: Run user settings when header is clicked