[apps/graph] add an attribute "activate" to functions

Change-Id: I1a1f548c540a6e31fe9ce1aeb742bf6a6292362c

apps/graph/function.cpp

@@ -12,7 +12,8 @@ Graph::Function::Function(const char * text, KDColor color) :
   m_color(color),
   m_name("f(x)"),
   m_expression(nullptr),
-  m_layout(nullptr)
+  m_layout(nullptr),
+  m_active(true)
 {
 }
 
@@ -47,3 +48,11 @@ ExpressionLayout * Graph::Function::layout() {
   }
   return m_layout;
 }
+
+bool Graph::Function::isActive() {
+  return m_active;
+}
+
+void Graph::Function::setActive(bool active) {
+  m_active = active;
+}