Tell user that added directories have to be scanned.
Only ask once for rescan at preference close

Only ask to rescan the library if the user is done editing the
preferences.
Show the rescan dialog also in the browsefeature

The warning dialog about the rescan is now also shown in the
browsefeature and the action to add another directory is disabled while
the library scanner is running.
Rename buttons to 'Scan'/'Cancel'

clean up the logic a little bit and remove the reinterpret cast as well.
Merge pull request #351 from kain88-de/add-dir-warning

Tell user that added directories have to be scanned.