Make wx and wxagg work with wx 2.9.x on Mac.

This affects the wx* backends on OSX only.  It removes Mac-specific
code that was letting the Frame manage the toolbar directly, without
using the Sizer.  There was a comment saying that this was needed
for OSX 10.3.  With wx 2.9.x, however, this method of handling
the toolbar does not work at all; no toolbar appears, and the
program can hang or otherwise behave erratically.  The Sizer
method, however, works.

The big question is whether the Sizer method works on the Mac
with wx 2.8.x.  If it does, then this can be merged.