Tell user that added directories have to be scanned.

https://bugs.launchpad.net/mixxx/+bug/1273333

I added a message box to tell the user that a rescan of the library is required. 

In the bug report we talked about starting the library scan when a new directory is added. But that would inhibit use from adding more directories until the scan is finished. This is because as far as I'm aware that we can't write to the database from two different threads at the same time. So I think until we have moved the database access to a another thread I think this the best we can do.
Maybe add a comment that this is a dupe string from DlgPrefLibrary and to make sure you update it in both places if you change. To lighten translation load we should make sure they stay identical.