Debugger: Rework type handlers to allow for custom selection.

TypeHandler:
- Add name field for presentation purposes. Adapt subclasses accordingly.

TypeHandlerRoster:
- Add methods to count and retrieve all type handlers for a given type,
and adjust CreateValueNode to allow for passing in an explicit handler.
Adjust callers accordingly.

VariablesViewState:
- Add helpers to store an explicitly chosen type handler for a node.

TypeHandlerMenuItem:
- ActionMenuItem subclass that takes care of reference management
for its contained type handler.

VariablesView:
- Add context menu for choosing type handlers if applicable. Implement
support for invoking said type handlers in a similar manner to explicit
typecasts.
- Adjust saving/restoring the view state so that hidden nodes are taken
into account as well. This is necessary since it may be the case that
the handler had to be applied to the hidden child rather than the visible
node (i.e. the BMessage handler when applied to a pointer to a BMessage).

All together, these changes allow choosing to switch between views of a type
when the Debugger has multiple handlers for it. For example, for BMessages
this allows switching between displaying the raw underlying structure vs
the decoded message content.

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.