Fix QML watch expressions for v8

Fix syncing of watch expressions with Qt Quick 2. For v8/v4 we're
utilizing the 'evaluate' command, which we have to re-send on every
change to get updates. We therefore now call synchronizeWatchers()
whenever the stack changes.

Task-number: QTCREATORBUG-13473
Change-Id: I7356e9518a719839c5cbb6e518be18e665078e85
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>