Enabled the use of Debugger-specific watch/locals models.

- Modified WatchModel to handle storage of an hierarchy
  of WatchItems.
- Factored out code for asynchronous population to
  AsyncWatchModel and added a mixin for convenient
  handling
- Added base class for synchronous models.
- Implement simple, synchronous models for CDB, greatly
  simplifying code and finally getting manual
  expansion right.
Signed-off-by: hjk <qtc-committer@nokia.com>