VCS[CVS]: Complement menu options, support 'edit'.

Add missing menu options for project/repository actions. Add support for
'edit' (current file) and 'unedit' (current file/repository, with
prompt).
Note: Unediting modified files will only work with cvsnt, since it uses
the '-y' option to force a 'yes' to CVS asking to revert changes.
Implement IVersionControl::vcsOpen() by using 'edit', causing it to
prompt for read-only as it does for Perforce.

Task-number: QTCREATORBUG-1971
Task-number: QTCREATORBUG-1864