Debugger: Fix team restart request.

UserInterface:
- Add Clone() function to set of required virtuals. This asks the subclass
to create a new instance of its respective type.

{CommandLine,Graphical,Report}UserInterface:
- Implement the above function.

TeamDebugger:
- Add accessor for the currently active UI.

TargetHostInterface:
- Set correct request type when setting up the options for a team restart.
- Ask the TeamDebugger for its user interface and clone it in order to fill
in that aspect of the debug options. This fixes a regression introduced in
commit 880a64, which inadvertently resulted in team restarts no longer
working.

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.