[SC-IDE] save or discard changes on quit per unsaved document

[miguel-negrao]

When closing the program, it should iterate through all unsaved documents, presenting a dialog window asking if the user wants to save the changes or discard changes for each document. Standard behaviour in OSX, and probably other oses...

Currently we have to manually go to each unsaved document and save it. Also in OSX, since there is no indication of which documents are saved and which aren't we have to go document by document.

[jleben]

Implemented in cc14c61