Add new MessageBox class for showing short messages in a modal window.

Also:
- Enhance Editor to show error message box when saving action fails.
- Cleanup DefaultStyle.xml and at the same time create more common styles for reuse.
- Remove unused member variable in FileSelector class.
- ListView class does not assume vertical layout of its items. In theory we could have horizontal layout items if it is being styled as such.
- Prevent UIElement XML serialization to save invalid "UIElement" style to the final output. Fix invalid style (if already exist in the loaded UI layout) to cause error in attribute filtering when serializing.