COMP: Moved subject hierarchy base plugin classes in widgets

In order to subject hierarchy plugins be accessible from extensions, the include directories need to be exported. So that the current infrastructure can be used, all subject hierarchy plugin classes (abstract, handler, core plugins not related to modules) were moved from Plugins to Widgets. Plugins subdirectory thus became unnecessary and removed.
COMP: Fixed Mac and Linux build error

Subject hierarchy widget designer plugins were deployed to Slicer_DIR/bin, which broke the Linux and Mac build. This explicit output directory setting has been removed.