appDisplay: Factor out shared code into BaseAppView

appDisplay: Factor out shared code into BaseAppView

The two BaseAppView subclasses now share a lot in terms of
widgetry: they both have a scroll view, pagination dots, swipe
management, etc.

Move this shared code into BaseAppView. Notice, however, that
BaseAppView only creates the widgetry, but it doesn't add them
to any specific layout. FolderView arranges the widgetry in a
vertical box, while AppDisplay arranges it in a ShellStack.

Add a new 'orientation' parameter to BaseAppView and use it
to determine the orientation of the pagination dots, the swipe
tracker direction, and the scroll event handling.

It is worth noticing that the scroll event is a bit more
sophisticated now: when the orientation is horizontal, it
handles all directions since mice wheels usually only generate
up/down events.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1271