Support additional notebook/sourceview windows

[VJSchneid]

It would be a nice feature to support additional notebook/sourceview windows, which would make the work with multiple screens more comfortable.

I made a little effort already and if you want to I would finish the implementation.

[eidheim]

At first thought, it might be somewhat difficult to do this right, but it might be that supporting several notebooks is a good idea. The major changes in window.cc and its menu item handlers would be to receive the active notebook depending on which source view that was last focused. If you make it work, we are of course willing to accept such a pull request:)

[VJSchneid]

I will give it a try ;)