Change exit dialog to ask for confirmation to apply previewed background...

See [Issue #32](https://github.com/l3ib/nitrogen/issues/32)
This changes the exit dialog from this:
![2013-07-20-191705_507x124_scrot](https://f.cloud.github.com/assets/1538657/831210/2ccaa4ae-f1b7-11e2-8a6f-54ff026009d0.png)
to this:
![2013-07-20-231236_456x124_scrot](https://f.cloud.github.com/assets/1538657/831209/224c185a-f1b7-11e2-95fd-c2c666c59c6f.png)

The Cancel button is redundant with the window delete event, since they both close the dialog and do not exit Nitrogen. I wouldn't be opposed to removing the Cancel button.

The No button continues quitting without applying.
The Yes button continues quitting after applying.

This involved decoupling the apply function from the sighandle_click_apply into a new protected member function apply_bg.