Change the way applications should be started.
Add a new variable *qapplication*.
make-qapplication creates a new QApplication if *qapplication* is NIL,
otherwise it returns *qapplication*.
Remove the QT-USER package, which had similar functionality, but
there's no reason to keep this separate.
Add a with-main-window macro.
Remove enable-syntax macro for (named-readtables:in-readtable :qt).
enable-syntax is not a very descriptive name, and
(named-readtables:in-readtable :qt) is not much longer.