Change exit dialog to ask for confirmation to apply previewed background.
On button press "No" in exit dialog, reset background to that stored in the configuration file.
Merge branch 'better_dialog' of https://github.com/jameh/nitrogen into jameh-better_dialog
Fix method rename from setter refactor
Added ability to restore bg set before nitrogen ran

NOTE: this only works with a single display like a Xinerama full screen

On first background set inside of nitrogen, the original pixmap set is saved
instead of being destroyed.  At program exit, it is either restored or
destroyed as appropriate.

- Fixes keyboard shortcuts not triggering save prompt
- Factored out several pieces of SetBG functionality into helper methods
Disable pixmap saving for non-interactive modes
Initial pixmaps saved per display
Copy initial pixmap, fixes individual screen in Xinerama issue