QmlProfiler: Only load data into child models when it's ready

Loading data on ProcessingData would lead to incomplete information
about source code locations. Rebuilding the whole model on any kind
of changed() signal is wasteful.

Task-number: QTCREATORBUG-11100
Change-Id: I9f8d718bff6149ca2dc2d0482c11d66d940af15d
Reviewed-by: Kai Koehne <kai.koehne@digia.com>