Squashed 'modules/base/term_reference_tree/' content from commit cbf30ba

git-subtree-dir: modules/base/term_reference_tree
git-subtree-split: cbf30baca5c2a522779de8b66168f6a431189b3b
New: Add modules/base/term_reference_tree.
Merge commit '14d3d3a5b9e90009261e2deec917adbd57d5191d' as 'modules/base/term_reference_tree'