MouseEvent now keeps track of previous event and drag-start event (only implemented for Qt).
Added MouseEvent.drag_events() and .trail()
Fixed up Event.__repr__ to avoid recursion
Extend to support glut and pyglet
Renamed canvas backend methods to _vispy_*
updated backend tests to exclude new mouse methods.
MouseEvent now keeps track of previous event and drag-start event.
Added MouseEvent.drag_events() and .trail()
Fixed up Event.__repr__ to avoid recursion
Updated backend tests to exclude new mouse methods.

Merge branch 'mouse_events'