AppKit: BMessage Add/Find/Get/Has/ReplaceNodeRef

This allows you to pass node_ref's around like you can entry_ref's.

Added node_ref_flatten(), node_ref_unflatten() and node_ref_swap() to
MessageUtils. These are close cousins to entry_ref_flatten(),
entry_ref_unflatten(), and entry_ref_swap() but for node_ref's.

Added B_NODE_REF_TYPE to TypeConstants.h in the Support Kit.

Added B_NODE_REF_TYPE to Debugger and ByteOrder in Support Kit,
B_NODE_REF_TYPE is treated the same as a B_REF_TYPE (entry_ref).

Add documentation for new NodeRef methods and B_NODE_REF_TYPE.

Change-Id: I32c6ed276bf1a7894a835b9fc9de5a882c35883c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3182
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

Debugger: Fix #13939, more work on #13800.

- Fix various cases where OpenHashTables weren't being cleared properly.
- Fix various reference counting errors.
- Simplify FileManager reference handling.
- Fix bug in LocatableDirectory where the directory named '/' would have its
name returned as empty. This would lead to failed lookups for entries already
in the table, and ultimately corrupted the hash table when deleting unused
entries, leading to #13939. This was previously never noticed due to the
entries not being freed properly.
- AbbreviationTable wasn't clearing its entries.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.