app: also move the "draw brush outline" logic from brush to paint tool

So all paint tools honor the setting.

Add GimpPaintTool::get_outline() which either returns an outline, or
calls gimp_paint_tool_set_draw_cursor() and implement it in
GimpBrushTool and GimpInkTool. Handle all brush/circle/fallback
drawing in gimp_paint_tool_draw().