Return target on mouse:down (even for unselectable objects)

canvas 'mouse:down' event only returns selectable objects.
Clear selection for unselectable object

Now objects are found even they are unselectable - expand _shouldClearSelection for unselectable objects.
Don't call findTarget() - new in parameter target.
findTarget now retuns unselectable objects
Merge pull request #568 from Kienz/patch-1

Return unselectable objects in canvas 'mouse:down' event