Make IDocument::fileName a member with setter.

Instead of requiring subclasses to implement a method.
Also renames IDocument::rename to IDocument::setFileName,
since it doesn't really rename any files or such.

Change-Id: I1344025c24d2f74a6a983e04fb0a5245f1f37aad
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>