Debugger: Fix the way to register/add new debuggers

DebuggerItemManager::registerDebugger() should return the id
of the added debugger in order to use it when setting a kit's debugger
(addDebugger() creates and adds a copy of the passed
debugger object with a unique id).

The DebuggerKitInformation::setDebugger() method should set only already
existing and registered debuggers.

Task-number: QTCREATORBUG-10436

Change-Id: Icdcd1ed92aafe9eda44abf831aa9983dd6801980
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>