* Add split tab support for tree tabs
In order to support split tabs in tree tabs, mainly we need to reimplment
two methods - CreateSplit and Unsplit.
For CreateSplit, we need to wrap the split tabs in a tree node, and insert it
at the position.
For Unsplit, we need to extract the tabs back to the tree nodes.
- TabStripCollection: virtual CreateSplit, Unsplit, AddCollectionMapping,
RemoveCollectionMapping; AddTabCollectionAtPosition for delegate.
- BraveTabStripCollection(delegate): override CreateSplit/Unsplit and
AddCollectionMapping/RemoveCollectionMapping; PassKey overloads for
AddTabCollectionAtPosition and collection mapping.
- BraveTreeTabStripCollectionDelegate: implement CreateSplit (wrap split in
tree node, insert at position), Unsplit (extract tabs back to tree nodes),
AddCollectionMapping/RemoveCollectionMapping for TREE_NODE holding split/group;
MoveTabsRecursive same-position handling for split/group creation.
- Tab strip API: handle TREE_NODE in tab_converters (unpinned container).
- Docs: tree tabs + split tabs flow and MoveTabsRecursive behavior.
Sangwoo Ko
b7a3eeb712