Toggle-Drag UI Feature

Dragging on toggle buttons can now be used to press multiple buttons at once, especially useful for layer and outliner buttons.


notes:
- automatically enabled for all toggle buttons
  (may change this if it becomes a problem).
- only buttons of the same type are pressed
  (helps avoid annoyances eg; dragging past layer buttons onto other 3d header buttons and pressing by accident).
- automatic axis locking - dragging will lock to X/Y depending on the initial drag direction,
  makes swipe motions work better, especially with the outliner.


implementation details:
- may re-implement as a region handler (currently its a modal operator).
- checking buttons in-between cursor motion events could be more efficient (but currently works ok).
- button execution needs to be improved
  (currently executing a button thats not under the mouse needed a workaround for passing uiHandleButtonData),
  requires further changes to UI code, will do next.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54742 954f8c5b-7b00-dc11-b283-0030488c597c