2.5: UI Layout Engine, initial code.

2.5: UI Layout Engine, initial code.

* As a test, used by:
  * Object buttons, tried to make it match the mockup.
  * Text window header.
  * Text window properties panel.
* Panel interaction with view2d is still problematic, need to make
  this work properly still.
* Templates are very basic, the ones there are simple but already
  can follow the object buttons mockup quite closely.
* It's based on a three level system: panels, templates and items.
  To get an idea of what that means in practice, see:

http://wiki.blender.org/index.php/BlenderDev/Blender2.5/UI_LayoutEngine#Panels.2C_Templates_and_Items