QmlJS: Fix semantic and non-semantic QML error reporting.

The QML snapshot only ever contains valid Documents; to compile a list
of parser problems we need to get at the invalid documents.

To do that, the model manager now provides a Snapshot with up to date,
but potentially invalid documents. That should also be useful for other
things.

Change-Id: I67892f63771c221bf2fe2c2bf0240a0f4e523227
Reviewed-on: http://codereview.qt.nokia.com/3012


Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>