app: don't create a fallback crosshair in gimp_brush_tool_create_outline()

Instead, draw the crosshair in gimp_brush_tool_draw() if create_outline()
didn't return an item *and* if there is no brush tool cursor.

In gimp_source_tool_draw(), don't add an additional crosshair if
create_outline() returned an item.

This fixes both "no cursor at all" and "both outline and crosshair
shown" for certain prefs settings conbinations (bug #623734).