Define externally accessible contants with 'var' instead of 'const'

Just as we did with classes, define other constants that are (or
may be) used from other modules with 'var' to cut down on warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/ui/status/volume.js

@@ -12,7 +12,7 @@ const PanelMenu = imports.ui.panelMenu;
 const PopupMenu = imports.ui.popupMenu;
 const Slider = imports.ui.slider;
 
-const VOLUME_NOTIFY_ID = 1;
+var VOLUME_NOTIFY_ID = 1;
 
 // Each Gvc.MixerControl is a connection to PulseAudio,
 // so it's better to make it a singleton