MWReferenceDialog: Allow swapping of existing references for others

By disabling the selection button instead of hiding it.

Also make sure that there are other references before re-enabling it.

Bug: 65689
Change-Id: Ifb3d6ab6eacb71909eb13e3a72e971b49e2595c1
Merge "MWReferenceDialog: Allow swapping of existing references for others"