UI

UI
* Headers and menus can now be created in python.
* Replaced the uiMenuItem functions to create menus with equivalent
  uiItem functions using a layout, removing duplicated code.
* More uiItem functions are now exposed to python.
* The text editor header, panels and one of its menus are now created
  in space_text.py.
* Buttons window data context icon new changes depending on active
  object.

Issues
* Icons are not wrapped yet, hardcoded ints at the moment.
* The ID browse template is unfinished.