painting. Also system added a brush every time it found no paint brush in the system which is not what we would want. Solution: * Brush panel stays visible always, regardless of whether there is a brush or not. * We search for first available brush when we find no brush in paint struct instead of always generating a new one. * Generating and searching for a brush take a mode argument now. Needed some refactoring to users of BKE_paint_init as well. * Did some style cleanups for paint mode enums. Patch is big but it's mostly argument refactoring.