Change the build system to enable building with Qt 6

This was built with: -DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00

Had to include "fixx11h.h" to fix some new conflicts between CursorShape enum in X11 code and in Qt code, looks like it's an old problem, and luckily someone else figured out a fix for it.

All unit tests that pass when building with Qt5 pass with Qt6 except:

• These two fail because of accessing an out-of-bounds index into QByteArray, caused by the code in send_message_internal() in kxmessages.cpp:

  • kwindowsystem-kstartupinfo_unittest
  • kwindowsystem-kxmessages_unittest

• kwindowsystem-kkeyserver_x11_unittest, fails with: QFATAL : KKeyServerTest::decodeXcbEvent(CTRL_SHIFT_keypad_end) ASSERT: "rc <= 0xffff" in file /usr/include/qt6/QtCore/qchar.h, line 115