Menu System - Disable invalid menu items

[GoogleCodeExporter]

Disable invalid menu items at appropriate times.

For example, with no project loaded, it’s not possible to “Project Save” 
(it does nicely prompt the user in this case); however, if Project > Add 
Include Path is selected, then a “Project Save” is performed, it does 
nothing apparently… no prompt, no save, no dialog.  If the invalid menu items 
(like Add Include Path) are disallowed until the appropriate time, then this 
invalid state would never be reached.

Original issue reported on code.google.com by jmar...@parallax.com on 13 Oct 2013 at 8:11