LibGUI: Move AbstractTableView::keydown_event() down to TableView

We can't really share this stuff with TreeView anyway, since tables
and trees have very different spatial relationships between indexes.

Libraries/LibGUI/TableView.h

@@ -38,6 +38,7 @@ public:
 protected:
     TableView();
 
+    virtual void keydown_event(KeyEvent&) override;
     virtual void paint_event(PaintEvent&) override;
 };