PopupMessage can now be accepted by pressing enter (Proof of concept)

PopupMessage now can behave like a popup usually behaves and will be
closed by either pressing Ok or pressing enter.

My first reaction when i get a popup is to close it by pressing enter and that is the standard in most GUI enviroments AFAIK. And since i couldnt see that pyglet-gui had support for it i implemented a simple fix. Im not sure if the implementation matches the design of this library so i push this commit to get comments so i can improve the commit if needed.
selected -> selectable?
period in end of sentence.