Finding the active view item button should only happen when it's actually
necessary, since looping through all buttons and blocks is an expensive
operation. This patch limits the search a bit more, to left clicks (the only
case that is actually handled).
This improves drawing performance in the node editor slightly,
where this was a bottleneck.
Differential Revision: https://developer.blender.org/D16882
8b1edff6b5