Vcs: Add scrollbar to AddToVcsDialog

This makes it handle adding a long list of files more gracefully.

Task-number: QTCREATORBUG-8467
Change-Id: I54dc0e405295694b4ea46ea426397223eb931722
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>