VCS/Diff editors: Set readonly attributes correctly.

VCS/Diff editors: Set readonly attributes correctly.

Make VCS-generated editors read-only, enable editing
when opening a patch.
Make VCS-Editor non-read-only by default, add setter
for "Forced read-only" that makes it a temporary,
read-only file.

Task-number: QTCREATORBUG-1528
Reviewed-by: Thorbjorn Lindeijer <thorbjorn.lindeijer@nokia.com>