xcb: Single process (GUI application) with multiple X display connections.

XCB platform plugin contains unmaintained code for Multiple X Display Connections from a Single Process. Decide if we want to support this or no.

The current situation is:

• This code was introduced in 2011 as an experimental feature (https://codereview.qt-project.org/#/c/2103/) and have been untested/unmaintained ever since (as far as I can tell). It has never been documented in QGuiApplication under "Supported Command Line Options". The intended command line was: ./app -platform xcb:address:display

• There are no JIRA bugs where users would complain that this feature does not work and they would need it (I did not find any). I don't even expect it to work in its current state, where almost all functions care only about the "default" connection, and don't attempt to support multi-connection. See more about this bellow.

• Seeing this undocumented code in the platform plugin is distracting for new contributors.

• XCB plugin respects the "-display" command line and DISPLAY envvar. This is sufficient to get your windows on different X displays. This is not the same as single process being able to create windows on several X displays simultaneously, but the question is - who would need this exotic setup?

Testing on a localhost:

Prerequisite: Create a new user account, log into the newly created account. This will start another X display (:1). Run "xhost +" on that account.
Tip: Create a shortcut for " dm-tool switch-to-greeter", you can use this to quickly toggle between accounts.

1) From the default account (where X display is at :0) start the "qscreen" manual autotest (qtbase/tests/manual/qscreen);
./qscreen -platform xcb::1 // Windows are not painted properly
Try
QT_XCB_GL_INTEGRATION=none ./qscreen -platform xcb::1
Now windows are painted fine, but things like drag-and-drop and copy-and-paste* does not work.

*from Qt window to some other place. Clipboard is optimized to bypass X server when copy-pasting within Qt application, so that is not a proper test case here.

Testing over a network:

1) set xhost + on your development machine.
2) Now from another PC connect to your development machine:
ssh -X user@ip_address
3) ./qscreen -platform xcb::0 // This will create window on both machines. Over network "QT_XCB_GL_INTEGRATION=none" does not seem to be required for GUI to work, but other issues are still there.

Proposed patch to remove this stale code: https://codereview.qt-project.org/#/c/154756/