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
3/ Qt separates file name and app name with an em-dash. It seems that
   only KDE does that: Gnome does not, Windows does not either. I do not
   think that we can/want to detect a KDE environment at run-time.

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.

(cherry picked from commit 82808fea04315fd323ec074e8ad7865d190987d4)
(cherry picked from commit 9313f8565b41c9de4b1990e155fe3fe8468f3446)
(cherry picked from commit bbe36ce6ce87ae37b7c30c250efbff2f9c185de4)
(cherry picked from commit 7cbc0c16a841716fa8966e0f800ee2cdc0432e25)