Rename cleartextSize to cleartext-size in object notify

https://bugzilla.gnome.org/show_bug.cgi?id=719377
Fix namespace creation in DIDLLite parser
Check for empty mandatory tags during UpdateObject

https://bugzilla.gnome.org/show_bug.cgi?id=705522
Prepare 0.12.5
Post-release version bump

Signed-off-by: Jens Georg <mail@jensge.org>