[FEATURE] Option to build & install QCH file with the public API dox

Using the new extra-cmake-modules module ECMAddQch (since 5.36.0)
this adds the option to automatically build and install a file
in QCH format with the docs about the public API, which then can be
used e.g. in Qt Assistant, Qt Creator or KDevelop.

Additionally the installed cmake config files will be extended
with a target KF5Wayland_QCH containing information about how to "link"
into the generated QCH file, which then can be used in the cmake build
system of other libraries building on this library, by
simply listing this target in "LINK_QCHS" of their ecm_add_qch() usage.
And a respective doxygen tag file with all the metadata about the
generated QCH file and used for the "linking" will be created and
installed.

Pass -DBUILD_QCH=ON to cmake to enable this.

src/wayland/CMakeLists.txt

@@ -169,7 +169,7 @@ set_target_properties(KF5WaylandServer PROPERTIES VERSION   ${KWAYLAND_VERSION_S
 
 install(TARGETS KF5WaylandServer EXPORT KF5WaylandTargets ${KF5_INSTALL_TARGETS_DEFAULT_ARGS})
 
-install(FILES
+set(SERVER_LIB_HEADERS
   ${CMAKE_CURRENT_BINARY_DIR}/KWayland/Server/kwaylandserver_export.h
   blur_interface.h
   contrast_interface.h
@@ -210,9 +210,16 @@ install(FILES
   textinput_interface.h
   touch_interface.h
   xdgshell_interface.h
+)
+
+install(FILES
+  ${SERVER_LIB_HEADERS}
   DESTINATION ${KF5_INCLUDE_INSTALL_DIR}/KWayland/Server COMPONENT Devel
 )
 
+# make available to ecm_add_qch in parent folder
+set(KWaylandServer_APIDOX_SRCS ${SERVER_LIB_HEADERS} PARENT_SCOPE)
+
 include(ECMGeneratePriFile)
 ecm_generate_pri_file(BASE_NAME KWaylandServer LIB_NAME KF5WaylandServer DEPS "core" FILENAME_VAR PRI_FILENAME INCLUDE_INSTALL_DIR ${KDE_INSTALL_INCLUDEDIR_KF5})
 install(FILES ${PRI_FILENAME}

src/wayland/KF5WaylandConfig.cmake.in

@@ -4,6 +4,7 @@ include(CMakeFindDependencyMacro)
 find_dependency(Qt5Gui @REQUIRED_QT_VERSION@)
 
 include("${CMAKE_CURRENT_LIST_DIR}/KF5WaylandTargets.cmake")
+@PACKAGE_INCLUDE_QCHTARGETS@
 
 function(kwaylandtest testBinaryName)