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.

release/scripts/startup/bl_ui/space_image.py

@@ -705,7 +705,7 @@ class IMAGE_PT_tools_transform_uvs(Panel, UVToolsPanel):
         col.operator("transform.shear")
 
 
-class IMAGE_PT_paint(Panel, BrushButtonsPanel):
+class IMAGE_PT_paint(Panel, ImagePaintPanel):
     bl_label = "Paint"
     bl_category = "Tools"
 
@@ -721,6 +721,11 @@ class IMAGE_PT_paint(Panel, BrushButtonsPanel):
         if brush:
             brush_texpaint_common(self, context, layout, brush, settings)
 
+    @classmethod
+    def poll(cls, context):
+        sima = context.space_data
+        toolsettings = context.tool_settings.image_paint
+        return sima.show_paint
 
 class IMAGE_PT_tools_brush_overlay(BrushButtonsPanel, Panel):
     bl_label = "Overlay"