modalDialog: grab focus immediately, not after fade-in

If the user types Alt+F2 and then immediately starts typing, some keys
can get lost. Fix that by grabbing focus sooner.

https://bugzilla.gnome.org/show_bug.cgi?id=644857

js/ui/runDialog.js

@@ -210,10 +210,7 @@ __proto__: ModalDialog.ModalDialog.prototype,
 
         this._entryText = entry.clutter_text;
         this.contentLayout.add(entry, { y_align: St.Align.START });
-        this.connect('opened',
-                     Lang.bind(this, function() {
-                         this._entryText.grab_key_focus();
-                     }));
+        this.setInitialKeyFocus(this._entryText);
 
         this._errorBox = new St.BoxLayout({ style_class: 'run-dialog-error-box' });