An important reorg and simplification of style-sheet-related code.  It should be safe.  All tests pass.
- Moved all border-related code to the attic.
  Contrary to comments, this code was being called, but it is not needed now that the style sheets specify border style.
  Gui classes no longer contain add_border and remove_border methods.
  As a result, the eventFilter method has fewer odious special cases.
- Added c.styleSheetManager ivar.
  Code in the qt plugins can assume that this ivar exists.  All other code must use hasattr to test whether it exists.
- Moved all stylesheet-related wrappers from LeoQtGui class to the attic.
  Instead of these wrappers, code calls the corresponding c.styleSheetManager class.  less cruft and indirection.
- Removed the call that sets the default color of a nested_splitter to "green".  This was quite confusing during testing ;-)

Leo build: 20140915192124