[solver] Move Poincare::Equal::solve() to Solver app to consider system

instead of single expression

apps/solver/equation.cpp

@@ -0,0 +1,45 @@
+#include "equation.h"
+
+using namespace Poincare;
+
+namespace Solver {
+
+Equation::Equation() :
+  Shared::ExpressionModel(),
+  m_standardForm(nullptr)
+{
+}
+
+Equation& Equation::operator=(const Equation& other) {
+  Shared::ExpressionModel::operator=(other);
+  return *this;
+}
+
+Equation::~Equation() {
+  if (m_standardForm) {
+    delete m_standardForm;
+    m_standardForm = nullptr;
+  }
+}
+
+void Equation::setContent(const char * c) {
+  tidy();
+  ExpressionModel::setContent(c);
+}
+
+void Equation::tidy() {
+  ExpressionModel::tidy();
+  if (m_standardForm) {
+    delete m_standardForm;
+    m_standardForm = nullptr;
+  }
+}
+
+Expression * Equation::standardForm(Context * context) const {
+  if (m_standardForm == nullptr) {
+    m_standardForm = static_cast<const Equal *>(expression(context))->standardEquation(*context);
+  }
+  return m_standardForm;
+}
+
+}