[escher/apps] Factorization of Toolbox.

MathToolbox methods moved to its parent, Toolbox (in escher).
Node and ToolboxNode classes renamed MessageTree and ToolboxMessageTree.
They are now in escher/, not in apps/.

Change-Id: Ibb26b0e9016094cef79506a1e9fc6dfb492d7103

escher/src/message_tree.cpp

@@ -0,0 +1,13 @@
+#include <escher/message_tree.h>
+
+int MessageTree::numberOfChildren() const {
+  return m_numberOfChildren;
+}
+
+I18n::Message MessageTree::label() const {
+  return m_label;
+}
+
+bool MessageTree::isNull() const {
+  return (m_label == (I18n::Message)0);
+}