Debugger: Fix #14375, #12343.

TeamDebugger:
- When a job is aborted, instead of calling into the user interface directly
to reset the status message, post a message to do so. Also, only post the
message if we aren't already in a terminating state. Otherwise, if jobs
were still running while the team debugger is executing its destructor, it
would attempt to make calls to the already destroyed user interface. This
bug has likely been with us for quite some time, but was hidden by incorrect
ref counting in the past (see #12343).

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: Fix a memory leak.

TeamDebugger:
- In several cases, BReferences on thread handlers after a call to
_GetThreadHandler() weren't correctly initialized to account for
that function implicitly acquiring a reference on our behalf, leading
to the reference count to spiral out of control, and consequently the
handler would never be freed. Found while attempting to address
#13800, likely more to come.

Debugger: Fix regression introduced in hrev50534.

FunctionInstance:
- Add new state FUNCTION_SOURCE_SUPPRESSED. This signals that the user
explicitly forced disassembly to be loaded despite source code being
available.

LoadSourceCodeJob:
- When forced to disassembly, use the above suppressed state accordingly.

SourceView/TeamWindow/TeamDebugger:
- Adjust to take new state into account as needed.

TeamDebugInfo::GetActiveSourceCode:
- When looking at a function to decide whether to return line information
based on source or disassembly, first examine the source code state. If
the source has never been loaded for that function, but we have it available,
set it on the function at that point. This lazily addresses the fact that
LoadSourceCodeJob is called on behalf of a specific function, and
consequently only sets the source code onto that function, and not all others
present in the same file. This allows us to differentiate between the case
where a function doesn't have source code available at all, versus a function
that has simply been forced to disassembly view at this point in time.

The primary symptom of the above issue was that attempting to set a breakpoint
outside of the currently active function, but within the same file would result
in the breakpoints view indicating that the breakpoint was at line 0 rather
than the appropriate line, and breakpoints would also not be drawn in the
source view for such locations.

Thanks to Humdinger for the heads up!

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.