QmlDesigner: Speedup copy and paste

Checking the possible imports can be slow and is not required
for copy and paste. Doing the same optimization for the style merger.

Change-Id: I7485a8f897db3d0fa595695bef10d315e93ff73e
Reviewed-by: Burak Hancerli <burak.hancerli@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/integration/designdocumentview.cpp

@@ -148,6 +148,7 @@ void DesignDocumentView::fromText(const QString &text)
 
     RewriterView rewriterView{externalDependencies()};
     rewriterView.setCheckSemanticErrors(false);
+    rewriterView.setPossibleImportsEnabled(false);
     rewriterView.setTextModifier(&modifier);
     inputModel->setRewriterView(&rewriterView);
 

src/plugins/qmldesigner/designercore/model/stylesheetmerger.cpp

@@ -555,6 +555,7 @@ void StylesheetMerger::styleMerge(const QString &qmlTemplateString,
     templateRewriterView->setTextModifier(&textModifierTemplate);
     templateModel->attachView(templateRewriterView.data());
     templateRewriterView->setCheckSemanticErrors(false);
+    templateRewriterView->setPossibleImportsEnabled(false);
 
     ModelNode templateRootNode = templateRewriterView->rootModelNode();
     QTC_ASSERT(templateRootNode.isValid(), return );