qt: add option to hide mouse cursor

Allow the user to build Qt with the QT_NO_QWS_CURSOR macro set.

Signed-off-by: Daniel Nyström <daniel.nystrom@timeterminal.se>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>