Add pinning approach for countries

[paetz]

If swiping to the right on a country list element a pinning icon should appear and the according country should be pinned to the top of the list.

• This must be stored in the SharedPreferences.
• Add a separator between pinned and unpinned countries (Maybe use an empty general item --> depending on Migrate RegionManager's list to RecyclerView #313)
  https://code.tutsplus.com/tutorials/adding-swipe-gestures-to-recyclerviews--cms-32427

Do not forget the possibility to unpin again :)

[paetz]

... or do it the other way around: Swiping means archiving countries/regions. Archived elements may be accessible via a button below the list.

[chrgernoe]

I would not implement the archive stuff because the list is not so long.