Add option to build and install the designer plugin

The patch adds a cmake option BUILD_DESIGNER_PLUGIN, targeted
mainly at package maintainers to allow an easy build and installation
of the FreeCAD designer plugin.

It has been discussed in
https://forum.freecadweb.org/viewtopic.php?f=10&t=67706

Signed-off-by: Bernd Waibel <waebbl-gentoo@posteo.net>

cMake/FreeCAD_Helpers/BuildAndInstallDesignerPlugin.cmake

@@ -0,0 +1,9 @@
+macro(BuildAndInstallDesignerPlugin)
+    # =================================================================
+    # ============= Build and install the designer plugin =============
+    # =================================================================
+
+    if(BUILD_DESIGNER_PLUGIN)
+        add_subdirectory(src/Tools/plugins/widget)
+    endif(BUILD_DESIGNER_PLUGIN)
+endmacro(BuildAndInstallDesignerPlugin)

cMake/FreeCAD_Helpers/InitializeFreeCADBuildOptions.cmake

@@ -80,6 +80,8 @@ macro(InitializeFreeCADBuildOptions)
     endif()
     configure_file(${CMAKE_SOURCE_DIR}/src/QtOpenGL.h.cmake ${CMAKE_BINARY_DIR}/src/QtOpenGL.h)
 
+    option(BUILD_DESIGNER_PLUGIN "Build and install the designer plugin" OFF)
+
     if(APPLE)
         option(FREECAD_CREATE_MAC_APP "Create app bundle on install" OFF)
 

cMake/FreeCAD_Helpers/PrintFinalReport.cmake

@@ -92,6 +92,11 @@ macro(PrintFinalReport)
             message(STATUS "Qt5WebKitWidgets:    not needed (BUILD_WEB)")
             message(STATUS "Qt5WebEngineWidgets:    not needed (BUILD_WEB)")
         endif(BUILD_WEB)
+        if(BUILD_DESIGNER_PLUGIN)
+            message(STATUS "Designer plugin:     ${DESIGNER_PLUGIN_LOCATION}/libFreeCAD_widgets.so")
+        else(BUILD_DESIGNER_PLUGIN)
+            message(STATUS "Designer plugin:     not built per user request (BUILD_DESIGNER_PLUGIN)")
+        endif(BUILD_DESIGNER_PLUGIN)
         if(${Qt5WinExtras_FOUND})
             message(STATUS "Qt5WinExtras:        ${Qt5WinExtras_VERSION}")
         endif()

cMake/FreeCAD_Helpers/SetupQt.cmake

@@ -29,6 +29,9 @@ if(BUILD_GUI)
             endif()
         endif()
     endif()
+    if(BUILD_DESIGNER_PLUGIN)
+        find_package(Qt5Designer REQUIRED)
+    endif()
     if(MSVC)
       find_package(Qt5WinExtras QUIET)
     endif()