Modify layer box hand cursor behavior

Fixes #127.

Basically, this limits the hand cursor to appearing only when the cursor
is over a valid layer (and not just over the layer box in general).

Thanks to @Kroc for catching.
Integrate audioglider's split-toning tool

It lives in the new Adjustments -> Photography menu.  (No menu icon yet;
icons are on hold pending a program-wide icon refresh for 6.4's
release.)