Default menus for fltk backend

[Kai Habel]

 We have all bits together to enable default menus now. The attached 
changeset adds deafault menus to all figures. Of course they are not 
shown for the gnuplot backend.
All menu entries should work, except for the Help menu, where I have 
added placeholder for an about entry. It would be nice if we could add 
some internet resources there as well (home, docs, bugs, chat(?), ...) 
but for that we need a generic browser function which must work on all 
platforms.

Before pushing such a change I would like to get some feedback.

Kai

Changeset.default_menu
Description: Text document