app: add gimp_paint_options_set_default_brush_size()

and use it globally instead of two different methods, one of which was
forgotten to be ported to the new aspect ratio range where 0.0 means
1:1. Add a FIXME comment in paint_tools.pdb where I think setting the
default size is a bug, see #721249.

(cherry picked from commit ef858453724f6d4ca105fc9daec038fdff358f30)