Add the isCommutative method to expression.

Change-Id: I657df96c5e24168ecd04470252549ed84eb2cfe3

poincare/include/poincare/commutative_operation.h

@@ -11,6 +11,7 @@ class CommutativeOperation : public Expression {
     int numberOfOperands() override;
     float approximate(Context& context) override;
     ExpressionLayout * createLayout() override;
+    bool isCommutative() override;
   protected:
     virtual float operateApproximatevelyOn(float a, float b) = 0;
     virtual char operatorChar() = 0;

poincare/include/poincare/expression.h

@@ -40,6 +40,7 @@ class Expression {
     Expression * simplify();
 
     virtual Type type() = 0;
+    virtual bool isCommutative();
 
     virtual float approximate(Context& context) = 0;
 };

poincare/src/commutative_operation.cpp

@@ -57,3 +57,8 @@ ExpressionLayout * CommutativeOperation::createLayout() {
   }
   return new HorizontalLayout(children_layouts, number_of_children);
 }
+
+bool CommutativeOperation::isCommutative() {
+  return true;
+}
+

poincare/src/expression.cpp

@@ -63,3 +63,7 @@ bool Expression::valueEquals(Expression * e) {
    * -riden. */
   return true;
 }
+
+bool Expression::isCommutative() {
+  return false;
+}