app: Reorder layer modes

This MR is more a suggestion because his changes are very subjective: the order of the GIMP layer list. The actual order appears to derive from some scientific reasoning, which I currently don't know.

This current order, however, is peculiar, with the lightening modes before the shading modes, among other things. This is different from the order used by our friend Inkscape and the mainstream Photoshop.

BEFORE MR	AFTER MR

Other software has an even weirder order: Krita and CorelDraw. What I mean by this is that there is no "correct" order. Therefore, if this MR is understood to be dispensable by the reviewer, it can be promptly closed.