[Bf-codereview] Limit brushes to a single paint tool (issue 5671088)

[NicholasBishop at gmail.com]

Fair point, uploaded a new patch that removes the new field and goes
back to the object mode enum.

The point of using a new C enumeration was really just to make clear
that it's choosing between brush tools (Brush.sculpt_tool,
Brush.vertextool, Brush.imagepaint_tool), but that's a bit weak anyway
since UV Sculpt doesn't have a brush tool field.

A couple other changes as well: removed the brush_paint_mode from struct
Paint, was really only used in one place and easily replaced.

Also changed paint_init to not require the brush type by not creating a
default brush if none exists. This situation would only normally come up
when creating a new Scene anyways, at which point the normal brush
selection UI is probably more useful than creating a new generic brush
from scratch.

http://codereview.appspot.com/5671088/