modal-dialog: Don't let buttons steal manually-set focus

ModalDialog provides a method to set the initial focus. However,
when adding buttons, the initial focus is always set to the last
button, thus overwriting a previously set manual focus.
Instead, only set the initial key focus if setInitialKeyFocus()
has not been called manually before.

https://bugzilla.gnome.org/show_bug.cgi?id=659133

js/ui/modalDialog.js

@@ -131,6 +131,8 @@ ModalDialog.prototype = {
             else
                 x_alignment = St.Align.MIDDLE;
 
+            if (this._initialKeyFocus == this._dialogLayout ||
+                this._buttonLayout.contains(this._initialKeyFocus))
                 this._initialKeyFocus = buttonInfo.button;
             this._buttonLayout.add(buttonInfo.button,
                                    { expand: true,