1. (Windows) Fix problem with defining buttons within SBX modules. Now they act like menu items, i.e., SBXs, and their parent programs share the same set of buttons and menus. Buttons or menus defined by an SBX remain active on return to the calling program.
2. (Windows) Maximum number of buttons allowed has been increased from 124 to 256.
3. Clean up error handling when an XFUNC() references a non-existent SBX. (Now aborts with unsupported function error.)
4. Fix a problem with nesting of XFUNC() calls within expressions. (The expression stack was not getting reset at the right time.)
