[apps/shared] Split ExpressionModelHandle into ExpressionModelHandle and

SingleExpressionModelHandle (in perspective of Sequence model who will
be composed of 3 ExpressionModelHandle)

apps/shared/single_expression_model_handle.cpp

@@ -0,0 +1,27 @@
+#include "single_expression_model_handle.h"
+#include "global_context.h"
+#include "poincare_helpers.h"
+#include <poincare/horizontal_layout.h>
+#include <string.h>
+#include <cmath>
+#include <assert.h>
+
+using namespace Ion;
+using namespace Poincare;
+
+namespace Shared {
+
+SingleExpressionModelHandle::SingleExpressionModelHandle(Storage::Record record) :
+  Storage::Record(record)
+{
+}
+
+bool SingleExpressionModelHandle::isDefined() {
+  return !isEmpty();
+}
+
+bool SingleExpressionModelHandle::isEmpty() {
+  return value().size <= metaDataSize();
+}
+
+}