FreeImage: patch to support 16-bit grayscale + 16-bit alpha files

Trying to support so many HDR variants is unpleasant, but at least I
could use libpng to do the heavy lifting on this.

I've also submitted the patch upstream; fingers crossed that they get to
it soon.
pdButtonToolbox: allow the caller to specify custom disabled/hover behavior

Some toolbox buttons need to be brightened when disabled; others do not.
Same for hovering.
Overhaul file and undo area of left toolbox

Everything but the "new image" button is implemented.  I'm still not
sure if that button is the best use of space, but that's okay - it's
easily changed.

The new "Save Lossless Copy" button is great for accelerating typical
workflows.  It's fully implemented, but I still need to add a
corresponding menu item to the File menu.

The new "Fade last" shortcut button is also a workflow lifesaver, IMO.
I use this feature constantly and believe it's worth calling attention
to.

Tooltips are not as pretty as the old ones, due to the lack of a
"tooltip title" feature.  I really need to write my own tooltip class
instead of tacking modifications onto my existing one, but not sure if
that'll happen this release cycle.