js: Use SOURCE_CONTINUE/SOURCE_REMOVE constants in source functions

With support for boolean constants in g-i, we can finally use the
more readable constants instead of true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/ui/endSessionDialog.js

@@ -411,7 +411,7 @@ const EndSessionDialog = new Lang.Class({
                 this._secondsLeft = this._totalSecondsToStayOpen - secondsElapsed;
                 if (this._secondsLeft > 0) {
                     this._sync();
-                    return true;
+                    return GLib.SOURCE_CONTINUE;
                 }
 
                 let dialogContent = DialogContent[this._type];
@@ -419,7 +419,7 @@ const EndSessionDialog = new Lang.Class({
                 this._confirm(button.signal);
                 this._timerId = 0;
 
-                return false;
+                return GLib.SOURCE_REMOVE;
             }));
     },