Present a list of files to save / discard (as with kdev3) on:
- application close
- mainwindow close (only those files which will actually close, ie. that only have views in that main window)
- reload all documents
- close all documents / close other documents
- mulitple mainwindow fixes, only operate on the active mainwindow

Fix crash on close, Core was being accessed after destruction, now gets destroyed after last mainwindow destructor has run.