QmlDesigner: Enable transient ScrollBars for StudioStyle

- Transient scroll bar is available for StudioStyle.
- Some changes are applied to slider style, because scrollBar and
sliders have some common usages within the style.

Task-number: QDS-9283
Change-Id: I7a8b7997cf4d20142a0524c4a071b93dfd06321d
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

src/plugins/texteditor/texteditor.cpp

@@ -66,6 +66,7 @@
 #include <utils/textutils.h>
 #include <utils/theme/theme.h>
 #include <utils/tooltip/tooltip.h>
+#include <utils/transientscroll.h>
 #include <utils/uncommentselection.h>
 
 #include <QAbstractTextDocumentLayout>
@@ -1144,6 +1145,7 @@ TextEditorWidget::TextEditorWidget(QWidget *parent)
     setLayoutDirection(Qt::LeftToRight);
     viewport()->setMouseTracking(true);
     setFrameStyle(QFrame::NoFrame);
+    TransientScrollAreaSupport::support(this);
 }
 
 void TextEditorWidget::setTextDocument(const QSharedPointer<TextDocument> &doc)