More MXML improvements

Improved performance of children type resolution, now it's way more
acceptable, although caching should still be added.
Added some more cases for valid children types (Vectors and IFactories).
Improved state-related attributes when auto-completing tags.
Improved parent tag resolution on broken XMLs.
No attribute completion on closing tags.
Fixed some indentation problems after merging.
Removed an unwanted break resulting of merging as well.