A major rewrite of the big-text buttons.
It remains to be seen how the new code impacts the related git bugs,
but there is no doubt that the new code is simpler and better:

- The big text is now shown along with warning text and warning buttons.
- Converted LeoBigTextDialog class to BigTextController class.
  The controller now *has* gui elements, instead of *being* a gui element.
  All methods have been revised. Several are gone.
- Simplified all the interface code in LeoTree.select and helpers.
  The only injected ivar is w.leo_big_text_controller.
- Added QTextEdit#bigtextwarning to the Qt stylesheet to make the warning background pin.

Leo build: 20141019131925