When closing application without merging files ask for confirmation

[floatas]

If user adds\removes pdf file from merge list and tries to close application ask confirmation. But if he merged all files and didn't change file list afterwards, don't ask.

Scenarion 1:

1. Add pdf file
2. Close application
3. Ask confirmation

Scenarion 2:

1. Add pdf files
2. Merge them
3. Close application
4. Don't ask for confirmation