Debugger: Further work on memory leak hunting.

TeamWindow:
- Properly delete stack frame selection entries in destructor.

TeamDebugger:
- The reference to TeamDebugInfo wasn't correctly initialized to own,
leading to TeamDebugInfo never being destroyed.

FunctionInstance:
- Also clear source code reference in destructor.

ImageDebugInfo:
- Release references to specific infos in destructor. These are acquired
on our behalf when loading the image info, but were never released,
leading to the latter never being freed.

DwarfManager:
- Release references to files in destructor.

FileManager:
- Release entries in various destructors.

Somewhat improves the situation in #13800, but is still far from complete,
as a thorough review of this nature hasn't ever really been done.

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.