canvas/toolgrid: Remove (unused) mouse press event tracking
canvas/toolgrid: Remove (unused) mouse press event tracking

Fix an `AttributeError: 'NoneType' object has no attribute 'underMouse'`
in the `ToolButtonEventListener.eventFilter`. The mouse press tracking
was not even used so the class was removed, merging the parts that were
used into `ToolGrid` itself.

cheral/orange3,cheral/orange3,cheral/orange3,cheral/orange3,cheral/orange3,cheral/orange3