#65 add -padx and -pady options to menubars

[Reinhard Max]

This patch adds -padx and -pady options to the [menu]
command. They replace the hardcoded values for the
spacing around the entries in menus used as menubars.
The defaults for this options are the same as the
hardcoded values before.

Maybe the same options could be used to replace the
also hardcoded MENU_MARGIN_WIDTH and
MENU_DIVIDER_HEIGHT (see top of tkUnixMenu.c), but they
need different defaults, therefore I left that out for
the moment.

[Reinhard Max]

-padx and -pady options for menubars