util.js: Lock the screen before switching to the greeter.

Move some relevant code out of the user applet - calling the
util.switchToGreeter will lock the screen first.

cinnamonEntry.js: Allow addContextMenu() to re-use an existing

PopupMenuManager.

If the St.Entry is inside of a popup menu (and *not* a modal dialog),
that menu's 'captured-event' handler can receive unintended events,
and react to focus loss, closing both the original popup and context
menu.

The original menu's manager can safely deal with multiple popups,
and ensure 'correct' behavior in this situation.

ref:
#13472

Silence some gir-2.0-related warnings.

Silence some gir-2.0-related warnings.