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.

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23242 3bd1e089-480b-0410-8dfb-8563597acbee
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.

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23243 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Fixed subject hierarchy core plugins self test

Which failed due to renaming of GetAssociatedDataNode to GetAssociatedNode in
vtkMRMLSubjectHierarchyNode. It has been renamed, because by making
vtkMRMLHierarchyNode::GetAssociatedNode virtual, it became possible to override
it in subject hierarchy so that it can handle nested associations.

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23244 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: SubjectHierarchy - Fix build warnings and indentation

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23245 3bd1e089-480b-0410-8dfb-8563597acbee