appchooser: Vertical layout plus categories

Add categories view and put that and the item list view in a
vertical layout.

I plan to do a horizontal layout much more similar to the old
AppChooser so using a StackView I can switch between them.
appchooser: Horizontal view with grid

Yet another menu layout, this one is much like the original
AppChooser layout.
appchooser: Add layout selector
appchooser: Run applications

Trigger model action to run clicked applications.
appchooser: Layout mirroring for list items when RTL is enabled
appchooser: Spacing after list view