LibGUI: Add ModelIndex::data(ModelRole)

This is a convenience API that makes accessing model data easier.

Libraries/LibGUI/Forward.h

@@ -92,6 +92,7 @@ class Widget;
 class Window;
 class WindowServerConnection;
 
+enum class ModelRole;
 enum class SortOrder;
 
 }

Libraries/LibGUI/ModelIndex.cpp

@@ -25,10 +25,20 @@
  */
 
 #include <AK/String.h>
-#include <LibGUI/ModelIndex.h>
+#include <LibGUI/Model.h>
+#include <LibGUI/Variant.h>
 
 namespace GUI {
 
+Variant ModelIndex::data(ModelRole role) const
+{
+    if (!is_valid())
+        return {};
+
+    ASSERT(model());
+    return model()->data(*this, role);
+}
+
 const LogStream& operator<<(const LogStream& stream, const ModelIndex& value)
 {
     if (value.internal_data())

Libraries/LibGUI/ModelIndex.h

@@ -28,6 +28,7 @@
 
 #include <AK/Traits.h>
 #include <LibGUI/Forward.h>
+#include <LibGUI/ModelRole.h>
 
 namespace GUI {
 
@@ -35,7 +36,7 @@ class ModelIndex {
     friend class Model;
 
 public:
-    ModelIndex() {}
+    ModelIndex() { }
 
     bool is_valid() const { return m_row != -1 && m_column != -1; }
     int row() const { return m_row; }
@@ -57,6 +58,8 @@ public:
 
     const Model* model() const { return m_model; }
 
+    Variant data(ModelRole = ModelRole::Display) const;
+
 private:
     ModelIndex(const Model& model, int row, int column, void* internal_data)
         : m_model(&model)