(Fcoordinates_in_window_p): Return `left-bitmap-area'

(Fcoordinates_in_window_p): Return `left-bitmap-area'
and `right-bitmap-area' if position is in the bitmap areas.  This
avoids an error when clicking on the bitmap areas.  Instead, they
are currently treated like clicks inside the window.
(coordinates_in_window): Return 5 and 6 for bitmap areas.
(Qleft_bitmap_area, Qright_bitmap_area): New.
(syms_of_window): Initialize new symbols.