Update shortcut label for Unmaximize menu entry.

Update shortcut label for Unmaximize menu entry.

The window menu Unmaximize entry had the Maximize shortcut key as its
label.  That's because the Maximize and Unmaximize menu options are in
fact the same single entry with different text depending on the window's
state.

It can, however, cause confusion if a window is maximized using one
of the "Other maximization" options such as Maximus.  Selecting the
Unmaximize entry from the window menu would indeed unmaximize the window
but pressing the listed shortcut key would not.

We now dynamically update the shortcut label so that it shows a key
which will actually unmaximize the window.  Thus the menu description
and shortcut action are now consistent.