Change font size in correct window using mouse wheel

Change font size in correct window using mouse wheel

* lisp/mwheel.el (mouse-wheel-follow-mouse): Doc fix.
(mouse-wheel--get-scroll-window): New function extracted from...
(mwheel-scroll): ...here.
(mouse-wheel-text-scale): New function to change face height in
the correct window, depending on the value of
'mouse-wheel-follows-mouse'.  (Bug#28182)
(mouse-wheel-mode): Bind 'mouse-wheel-text-scale' instead of
'text-scale-increase' and 'text-scale-decrease'.