xcb: Allow to disable XInput 2 support by setting QT_XCB_NO_XI2 env var

Use of XInput 2 may result in regressions. For example we call
XISelectEvents for each window every time a device plugs/unplugs.
It causes significant delays when there are many native windows.

Change-Id: I60b799bb667d0e4bca1f9c52cdaa07b04bcc749f
Task-number: QTBUG-57013
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbconnection.cpp

@@ -631,7 +631,8 @@ QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, bool canGra
     initializeXRender();
 #if defined(XCB_USE_XINPUT2)
     m_xi2Enabled = false;
-    initializeXInput2();
+    if (!qEnvironmentVariableIsSet("QT_XCB_NO_XI2"))
+        initializeXInput2();
 #endif
     initializeXShape();
     initializeXKB();