[apps] Implement InteractiveCurveViewController::isCursorVisible

It was a pure virtual method implemented in derived classes
  Shared::FunctionGraphController,
  Regression::GraphController.
InteractiveCurveViewRange does not need anymore to hold CurveViewCursor
* m_cursor.

apps/graph/app.cpp

@@ -23,7 +23,7 @@ const Image * App::Descriptor::icon() {
 App::Snapshot::Snapshot() :
   Shared::FunctionApp::Snapshot::Snapshot(),
   m_functionStore(),
-  m_graphRange(&m_cursor)
+  m_graphRange()
 {
 }