Use callbacks in QmlEnginePrivate::evaluate(). This separates the
four code paths through the machinery into three separate handlers
and one direct access to the console.

This also fixes a bug where items were put into 'debuggerCommands'
but attempted to be removed from 'updateLocalsAndWatchers'.

Introduce a QmlEngine::updateLocals similar to what the other
engines do. Let the frame() and assignValue() paths use it.
Keep track of pending changes and call notifyUpdateFinished
if and only if the pending lookup queues is empty.

Finally, remove some dead code.

Change-Id: I173a52911d0de994b849fc6ab4f52ef7f64a8ba5
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>