Fix T45258, impossible to select brush when removing it from 2d

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.

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