Do some caching of window title and related UI

Do some caching of window title and related UI

This should avoid performance problems related to the window update machinery.
Moreover this fixes a crash introduced by 82808fea when closing a file.

Note that GuiWorkArea::Private already had a read_only_ member, but it
was unused.

Also rename LyXVC::vcname() to LyXVC::vcstatus() since it now contains
directly the UI string to be shown.