Certain keybindings should continue to work even when a popup
menu is on screen. For instance, the keybinding for showing
the app menu and the keyinding for showing the calendar are
examples.

This is achieved by putting in place a special "POPUP" action
mode, whenever a popup menu is active.  This mode replaces
the (e.g., "NORMAL" or "OVERVIEW") action mode that was in place
for as long as the popup menu is active.

But those keybindings should not work when the user is at the
unlock dialog (which uses an action mode of "UNLOCK").

Unfortunately, since commit c79d24b6 they do.

This commit addresses the problem by forcing the action mode
to NONE at the unlock screen when popups are visible.

Closes #851