Debugger: Rework register handling

Use register names as handle, not their index in the view.
Store the raw real values, not some stringified version
as primary data.  Use subentries to break down bigger
registers into smaller entities. Also remember the
previous value of a register and show it in a tooltip.

Change-Id: I8ae3cc8766a7b211bc7cc827c734e5cf6060825c
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: hjk <hjk121@nokiamail.com>