QmlJS: Fix semantic info update triggering updates.

When a part of the semantic info update (such as a Link) triggers
QmlTextEditorWidget::updateSemanticInfo (for instance by requesting
QML plugin dumps) that update request was ignored because the *old*
semanticInfo revision was compared to the editor revision.

Instead, compare the *future* semantic info revision to check whether
an update would be accepted.

Also, instead of using the same snapshot, get a new one from the model
manager. Otherwise updates to libraryInfos won't be considered.

Change-Id: I0df6197bebce580129292aab5ca8cf24512a0fe7
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>