Display mousewheel hint on invalid use

For reasons I don't understand, random users keep asking me why
PhotoDemon doesn't support the mousewheel for zooming.  There must be
photo editors out there that don't adhere to the standard Ctrl + Wheel
convention for zooming???

To that end, if the user uses the mousewheel when the current image is
not scrollable, a message will appear that describes PD's mousewheel
conventions.

I debated allowing the mousewheel to control zoom if no scroll bars are
active, but the inconsistency that introduces would probably just make
the matter worse...
Improve contribution notes
Refactor a bunch of selection tool code

Not glamorous work, but it needed to be done.  The goal is to slowly
unify most tool handling under a few core functions; this is a step in
that direction.