Drop Qt4 support, add Qt6 (#267)

CMakeLists.txt

@@ -28,19 +28,19 @@ find_package(Boost REQUIRED thread system)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(OGRE_OV OGRE)
 
-# Qt 4 or 5
-if("${rviz_QT_VERSION}" VERSION_LESS "5")
-  find_package(Qt4 ${rviz_QT_VERSION} REQUIRED QtCore QtGui)
-  include(${QT_USE_FILE})
-  macro(qt_wrap_ui)
-    qt4_wrap_ui(${ARGN})
-  endmacro()
-else()
+# Qt 5 or 6
+if("${rviz_QT_VERSION}" VERSION_LESS "6")
   find_package(Qt5 ${rviz_QT_VERSION} REQUIRED Core Widgets)
   set(QT_LIBRARIES Qt5::Widgets)
   macro(qt_wrap_ui)
     qt5_wrap_ui(${ARGN})
   endmacro()
+else()
+  find_package(Qt6 ${rviz_QT_VERSION} REQUIRED Core Widgets)
+  set(QT_LIBRARIES Qt6::Widgets)
+  macro(qt_wrap_ui)
+    qt6_wrap_ui(${ARGN})
+  endmacro()
 endif()
 ## Prefer the Qt signals and slots to avoid defining "emit", "slots",
 ## etc because they can conflict with boost signals, so define QT_NO_KEYWORDS here.