[Qt] Qt6 new CBindings version

[zeljko]

Hi all,

I've just commited to trunk new CBindings version 6.2.3 for Qt6 (based 
on Qt-6.2 LTS).
New version exports QGuiApplication_x11Display() and 
QGuiApplication_xcbConnection() via QNativeInterface::QX11Application.
Such implementation avoids usage of private stuff - QX11Info in this 
case, and we use same descriptor as Qt lib created by X11OpenDisplay().
For those who want to play with xcb there's QGuiApplication_xcbConnection().
For details why it's implemented via cbindings see:
https://forum.lazarus.freepascal.org/index.php/topic,61237.msg462126.html#msg462126

zeljko