debugger: rework WatchModel

It's one model for all locals, watch, return, tooltip and inspector
data. This allows more streamlined code paths and better isolation
of the model data from the WatchHandler. WatchItems are now registered
in a hash indexed by iname, so inames can be used as the primary
handle to watch data in the WatchHandler interface.

Change-Id: Idac0a808b5d785307496d1de4198a1f2e9ce3880
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>