SCXML Editor: fix behavior of else and elseif tags

SCXML specify that the <if> tag contains elements separated by tags
<else/> and <elseif/>.
The SCXML Editor used to nest instruction inside the <else/> and
<elseif/> tags.
This patch implement the correct behavior, so each new inserted element
is appended inside the wrapping <if>.  Note that the new elements are
always inserted as last child of <if>.

Task-number: QTCREATORBUG-17674
Change-Id: I327941bbbd8b0cc04b0c26553257ccb2a24c8306
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>

src/plugins/scxmleditor/plugin_interface/sceneutils.cpp

@@ -414,6 +414,15 @@ ScxmlTag *addChild(ScxmlTag *tag, const QVariantMap &data, GraphicsScene *scene)
     return nullptr;
 }
 
+ScxmlTag *addSibling(ScxmlTag *tag, const QVariantMap &data, GraphicsScene *scene)
+{
+    TagType newTagType = (TagType)data.value(Constants::C_SCXMLTAG_TAGTYPE, 0).toInt();
+    if (newTagType >= UnknownTag) {
+        return addNewTag(tag->parentTag(), newTagType, scene);
+    }
+    return nullptr;
+}
+
 } // namespace SceneUtils
 } // namespace PluginInterface
 } // namespace ScxmlEditor