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>

CMakeLists.txt

@@ -87,6 +87,8 @@ SetGlobalCompilerAndLinkerSettings()
 add_subdirectory(src)
 add_subdirectory(data)
 
+BuildAndInstallDesignerPlugin()
+
 CreatePackagingTargets()
 
 PrintFinalReport()