Make vtkContextScene ButtonPress/ReleaseEvent invoke a button event.

This event might be catched (for instance in PV) to provide interaction
with items.

Change-Id: I3ff3f9a4a121914faebcf6e5d87afc6bc886fe5f
Merge topic 'add_context_scene_button_release_event' into master

c80a4db2 Make vtkContextScene ButtonPress/ReleaseEvent invoke a button event.