The Fullscreen/Normal menu items in the main window are now checkable.

This further brings sanity to the Perspectives menu, as the Fullscreen and Normal menu items indicate which is selected.

Just thinking about this now, we could've replaced both of these menu items with a single menu item that is either "Enter fullscreen" or "Exit fullscreen", as that would likely take less space, but I guess since this is already done, it's at least a step in the right direction.