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/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)