implement base activity to handle actionbar navigation

navigation via the actionbar is handled in a base activity that
every activity which extended android.app.activity (except the parent
activity) now extends. duplication of shared code is thereby avoided
except in activities that extends other activity types.

the actual movement is done with Intent.FLAG_ACTIVITY_SINGLE_TOP
to avoid recreation of the parent activity.
update translations. add italian and serbian translations.

thanks to operationDIE for the serbian translation.
thanks to Marco Bonifacio (mbonix) for the italian translation.
move language preference to top

the language preference is probably better placed at the top
since it will be easier to find there if for some reason the
language has been changed to one that the user doesn't know.