Deluxe-paint-like skin

Here's a modification of Jamon's DP skin, attempting to make it look more like the original Amiga version of Deluxe Paint. When Jamon did it, Grafx2 had much less possibility of customizations, now I could match the button behavior more closely : Buttons are mostly black on a solid color, with only a lightened angle; and the 'selected' version of buttons is in negative. At first I found the blackened icons too dark, but after all it's really not bad, handy for immediately seeing which tools are active. Skin format suitable for Grafx2 2.3wip and later.