events they don't want to handle to stdio

instance variables.

bgenObjectDefinition.py: support chaining of object definitions
macsupport.py: support functions returning pointers to existing windows/dialogs
bgen.py: tighten rules for recognizing simple symbol definitions

- Added possibility to turn off mainloop event handling
- Check interrupts even when not handling events
- Be more carefulabout which events we want
- Use different method to determine fg/bg