Add Interface menu in MantidPlot and improve custom menu

[Mathieu Doucet]

Requirements discussed at the Mantid developer meeting:

• Add an Interface menu to MantidPlot.
  
• Add a dialog to let the user choose which entries they want in the interface menu.
  
• For each entry in the interface menu, allow for a setup.py script to be run when the item is added.
  
• Pop up the interface menu dialog when the installer runs.
  
• Keep option to add custom menu item to Interface.
  

[Mathieu Doucet]

(In [6864]) Added Interfaces top menu, filled with current Qt custom interfaces. Menu is now modifiable through Manage Custom Menus. Re #1736

[Mathieu Doucet]

(In [6881]) Modified SANS UI start script for new Interfaces menu. Re #1736

[Mathieu Doucet]

(In [6934]) Added PyQt interfaces to the Interfaces menu. Re #1736

[Mathieu Doucet]

Requirements and solution are http://www.mantidproject.org/MantidPlot_Interface_Menu

[Mathieu Doucet]

Works as described on the requirements page above.

[Michael Whitty]

works as described on the wiki in r[7139].

[Stuart Campbell]

This ticket has been transferred to github issue ​2583