QmlDebugging: Fix Watched Expressions

The data response was incorrectly parsed for watched expressions.
This patch fixes the parsing. It also fixes removal and readdition
of a watched expression.

Change-Id: I1a9e2db3d4963d7fe12ed468ae5466a106011487
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>