Removes dependency on QtX11Extras for Qt6.2-beta (#331)

* Removes dependency on QtX11Extras and implements placeholder solution with QtGUI private API

* Fixes CMakeLists.txt to work with Qt5 and Qt6

* Removes all other uses of X11extras

* Renames and moves x11 methods into ads::internal

* Fixes typo

* Adds gui-private to src.pro

.github/workflows/linux-builds.yml

@@ -16,7 +16,6 @@ jobs:
       run: |
         sudo apt-get update --fix-missing
         sudo apt-get install qt5-default
-        sudo apt-get install libqt5x11extras5-dev
     - name: qmake
       run: qmake
     - name: make