constrained/simultaneous fit menu option GUI problem

[butler]

Loading a fit into SasView? should activate the constrained fit menu item (which is greyed out when no fit is available). It however can be activated by switching to another perspective (Pr or invariant) and back. Also it will activate after loading another fit panel though sometimes you actually need to press fit on that panel to get it to activate.

Once activated it will stay active and work as expected even if all other fit panels are killed.

There is a bigger question about whether constrained fit should be allowed currently as polydispersity does not work in that instance but the GUI problem needs fixing either to forbid a constrained fit (though that should be easily changed once we fix the polydispersity issue) or always active when only one fit panel is up (not randomly active as it is now)