plugins/vscui: replace AdwClamp with AdwPreferencesPage

I replaced AdwClamp with AdwPreferencesPage, removed GtkBox which was holding all widgets and added AdwPreferencesGroup on top of GtkStack.

One thing I dont like is that the GtkStack part is looking like this now:

AdwPreferencesGroup
  GtkStack
    GtkStackPage
      AdwPreferencesGroup
    GtkStackPage
      AdwPreferencesGroup

AdwPreferencesGroup inside another AdwPreferencesGroup looking a bit strange but it works :)

Fixes: #1926 (closed)