Get rid of ugly wxDynamicCastThis()s in wxMSW wxControl code and add a
new virtual method overridden in wx{Tree,List}Ctrl instead.
Also stop comparing the font with wxSYS_DEFAULT_GUI_FONT, there doesn't
seem to be any good reason to set this particular font for these
controls neither.
In addition to simplifying and de-ugligying the code, this commit
incidentally fixes -Wnonnull-compare warnings in this code from gcc6
too.
c45a8d49f1