[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/toolbox_message_tree.cpp

@@ -0,0 +1,13 @@
+#include <escher/toolbox_message_tree.h>
+
+I18n::Message ToolboxMessageTree::text() const {
+  return m_text;
+}
+
+I18n::Message ToolboxMessageTree::insertedText() const {
+  return m_insertedText;
+}
+
+const MessageTree * ToolboxMessageTree::children(int index) const {
+  return &m_children[index];
+}