Fix tab-bar scrolling for mice that report mouse-wheel events

Fix tab-bar scrolling for mice that report mouse-wheel events

* src/keyboard.c (make_lispy_position): Call
'window_from_coordinates' with last 2 arguments non-zero, to have
it report on tool-bar and tab-bar positions.  Tweak the return
value according to the expectations of 'make_lispy_event'.
(make_lispy_event): No more need to inject "tab-bar" into a click
event on the tab bar: it's already there.