Meta::Album: introduce canUpdateCompilation(), setCompilation()

and convert subclasses to use these. Many many subclasses already had
equivalent methods, this reduces code duplication and eliminates one
static_cast. Some subclasses had equivalent methods but never used
them, these are deleted in order to avoid dead code.

This is a prerequisite for next commit that unifies album actions
across collections. This is rather a bigger change, so I'm CCing
list - if anyone opposes, raise your voice.

CCMAIL: amarok-devel@kde.org