LibGUI: Add GAbstractView base class for GTableView.

This is in preparation for adding a new view class.

LibGUI/GAbstractView.cpp

@@ -0,0 +1,35 @@
+#include <LibGUI/GAbstractView.h>
+#include <LibGUI/GModel.h>
+#include <LibGUI/GScrollBar.h>
+#include <SharedGraphics/Painter.h>
+#include <Kernel/KeyCode.h>
+
+GAbstractView::GAbstractView(GWidget* parent)
+    : GScrollableWidget(parent)
+{
+}
+
+GAbstractView::~GAbstractView()
+{
+}
+
+void GAbstractView::set_model(RetainPtr<GModel>&& model)
+{
+    if (model.ptr() == m_model.ptr())
+        return;
+    if (m_model)
+        m_model->unregister_view(Badge<GAbstractView>(), *this);
+    m_model = move(model);
+    if (m_model)
+        m_model->register_view(Badge<GAbstractView>(), *this);
+    did_update_model();
+}
+
+void GAbstractView::model_notification(const GModelNotification&)
+{
+}
+
+void GAbstractView::did_update_model()
+{
+    model_notification(GModelNotification(GModelNotification::ModelUpdated));
+}