Debugger: Don't register identical debuggers (unless forced)

User code can decide whether it wants something added unconditionally
(by creating a DebuggerItem with valid id), or let the manager re-use
something matching or create an id.

Task-number: QTCREATORBUG-10641
Change-Id: I4f5a1fbe881932ba5608f5c6116b4ea8352f8903
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>