LibGUI: Add a GModelNotification class that views will receive.

I don't want to use GEvent here since these need to be synchronous
and mixing sync and async GEvents would be stupid.

LibGUI/GTableModel.h

@@ -10,6 +10,27 @@
 
 class GTableView;
 
+class GModelNotification {
+public:
+    enum Type {
+        Invalid = 0,
+        ModelUpdated,
+    };
+
+    explicit GModelNotification(Type type, const GModelIndex& index = GModelIndex())
+        : m_type(type)
+        , m_index(index)
+    {
+    }
+
+    Type type() const { return m_type; }
+    GModelIndex index() const { return m_index; }
+
+private:
+    Type m_type { Invalid };
+    GModelIndex m_index;
+};
+
 class GTableModel {
 public:
     struct ColumnMetadata {