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.
I tried that at first. But the compiler is complaining and thinks that I need to cast it. It surprised me too because the auto-cast is working in dlgpreflibrary.cpp