Ask for rescan upgrade

https://bugs.launchpad.net/mixxx/+bug/1390019

Tell the user that we have to rescan the library for coverart when they are upgrading from 1.11.

It would be nice if a native speaker could look over the text that is displayed in the QMessageBox in upgrade.cpp
"Mixxx now supports displaying cover art.  Do you want to scan your library for cover files now?"

[Scan] [Not Now]