MultiSelectionMenu

Hello!

I have added the first version of the MultiSelectionMenu widget. In addition to the "AllowMultipleSelections" option in the constructor, I have also added a "AlwaysHaveASelection" option which ensures that at least one item is always selected. Following our discussion of the setSelected() method, it now accepts a list. Similarly, getSelected() will return a list. If a singular item is passed to setSelected, it is appended to the selection. Would you prefer that this functionality is moved to a different method? Perhaps called addToSelected?

I have already ported the EnumWidget and the FormatValuePlugWidget to use the MultiSelectionMenu (rather than the SelectionMenu) which I will be committing next.

Luke