ui: create a RoundRectItem class

Factor out code from ProfileWidget's ToolTipItem, but make
the radius of the corners dynamic. Move into backend-shared,
though a new ui-shared might be preferred.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

backend-shared/roundrectitem.h

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef ROUNDRECTITEM_H
+#define ROUNDRECTITEM_H
+
+#include <QGraphicsRectItem>
+
+class RoundRectItem : public QGraphicsRectItem {
+public:
+	RoundRectItem(double radius, QGraphicsItem *parent);
+private:
+	void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
+	double radius;
+};
+
+#endif