Qt apps on X11 do not accept the -display parameter

Qt5 apps should allow for well known platform command line parameters like "display", "geometry", "name", "class" (all X11). These parameters need to be passed on to the platform plugin to handle.
Users expect these parameters to work. The list given above works with all Qt4 and GTK applications and most other X applications in existence (sometimes with only one leading dash) and Qt5 apps should also support them.
This bug is not about having the platform plugin actually do something with those parameters (QTBUG-27349 covers that).

Original description:

Qt apps on X11 no longer support the "-display" option.

It would be nice to have it back, the functionality seems to be there, but "-platform xcb:host:server" is hard to discover.