[solver] Delete equation at first backspace event (instead of clearing

the expression as in Graph/Sequence)

apps/shared/expression_model.h

@@ -20,6 +20,9 @@ public:
   Poincare::ExpressionLayout * layout();
   virtual bool isDefined();
   virtual bool isEmpty();
+  virtual bool shouldBeClearedBeforeRemove() {
+    return !isEmpty();
+  }
   virtual void setContent(const char * c);
   virtual void tidy();
 private:

apps/shared/expression_model_list_controller.cpp

@@ -70,7 +70,7 @@ bool ExpressionModelListController::handleEventOnExpression(Ion::Events::Event e
   }
   if (event == Ion::Events::Backspace && !isAddEmptyRow(selectedRow())) {
     ExpressionModel * model = modelStore()->modelAtIndex(modelIndexForRow(selectedRow()));
-    if (!model->isEmpty()) {
+    if (model->shouldBeClearedBeforeRemove()) {
       reinitExpression(model);
     } else {
       if (removeModelRow(model)) {

apps/solver/equation.h

@@ -15,6 +15,9 @@ public:
   Equation(Equation&& other) = delete;
   void setContent(const char * c) override;
   void tidy() override;
+  bool shouldBeClearedBeforeRemove() override {
+    return false;
+  }
   Poincare::Expression * standardForm(Poincare::Context * context) const;
 private:
   mutable Poincare::Expression * m_standardForm;