Set window title according to platform UI

Set window title according to platform UI

The window title is built from the current file name and its
mofidication state. We use our own code instead of the automatic title
bar provided when windowFileName() is set because

1/ Qt does not keep the full path name
2/ Qt does not yield a nice application name

The "read only" and "version control" status are shown in the status bar:

* for read only we use the tab read only emblem (with the right size)
* for version control, we show the name of the backend (using a new
  vcname() method of the backend).

The iconText() of the view is not updated anymore, since this is
deprecated in Qt5.