Swap "Clear All and Fill All"

As a new map-maker, the BG/FG interaction was a little difficult to figure out at first but I got the hang of it. One thing that contributed to my confusion was that the "Add" button for Mask tool was on the left, but the "Fill" (or "Add All") was on the right. Vice versa for "Subtract" and "Clear", they are opposite from one another. Not a huge change, but it was a bit of a stumbling block for me at first.