pdInput: add Click event report parameter to MouseUp

This allows the owner to determine if a MouseUp event is also firing a
Click event.  If the Click event is being fired simultaneously, and it
is the preferred way to handle an action, the user can thus check the
new parameter, and avoid handling MouseUp at all.
Layer toolbox: implement drag/drop layer reordering

...(knock on wood)...  This proved much easier than I expected.

Drag/drop feedback should actually be better than other software,
because instead of waiting for MouseUp to finalize the move, PhotoDemon
provides a live preview of the composite image as you drag.  I think our
viewport renderer is awesome enough to do this in a responsive way, so
I'm running with it.

Also note that drag/drop reordering integrates nicely with the Undo/Redo
engine.  (The new Undo engine is proving itself indispensable with all
these layer-based features!  Reordering only requires an image header
backup, so it's extremely fast and lightweight).

Animation for the rearranging layers would make this even better, but
I'm afraid I won't tackle that until I have a standardized framework for
UI animation (as it would be a hideous amount of custom code otherwise).
But for now, I still think our current approach is pretty great,
especially compared to its FOSS photo editing peers.