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

Commit [86ea08b](https://github.com/jameh/nitrogen/commit/86ea08b5f7ad0363e942c89f137352ad0a1c84dd) addresses #32.
Commit [319421e](https://github.com/jameh/nitrogen/commit/319421eddea317327befb7654a4b2def8e4a63a4) addresses #34.

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 (resetting background to configuration file setting with `Util::restore_saved_bgs()`).
The Yes button continues quitting after applying (by calling `NWindow::apply_bg()`).

This involved decoupling the apply function from the `sighandle_click_apply()` into a new protected member function `apply_bg()`.