Menu items can now be marked as disabled. When disabled, they can still

be selected for menu navigation or on mouse over, but no actions will be

performed. Clicking or pressing Space or Enter will do nothing, and the

menu will remain open. This allows for accessible keyboard navigation

without risk of performing unwanted actions.
An internal change to state tracking had to be made. Previously,

#items tracked only menu items (no separators), but #itemViews

tracked all views (menu items and separators). This meant that an index

for one didn't apply to the other. We now keep those two consistent on

menu items, and track everything under #allItemViews.

Tested with a menu consisting of enabled and disabled menu items of

all types. Verified that I could select but could not invoke any disabled

item actions through any means.
Unit tests pass.