sound applet: Initialize this._playerIcon and simplify some code.

Fixes some warnings when a player is first added.

ref: 95ecab0

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.