Further Refactoring of Text Implementation

The CombatMenubar now handles all text displayed on it (Selected Agent,
preparation for highlighted Agent/Tile)
Menus no longer require a background texture, menus without background
textures function as normal menus
Replaced previous TextClass implementation with the StaticTextClass
Implemented TimedTextClass to allow for extension of the TextClass where
the sentence text may need to be preserved over a setence's life and
where the lifetime of the sentence may result in specific behaviour
- Implemented expiring error messages as the ErrorTextClass subclass