LibGUI: Add AbstractView "edit triggers" to improve editing control

This API allows the embedder of a view to decide which actions upon
the view will begin editing the current item.

To maintain the old behavior, we will begin editing when an item is
either double-clicked, or when the "edit key" (return) is pressed.

Libraries/LibGUI/AbstractTableView.cpp

@@ -248,8 +248,10 @@ void AbstractTableView::doubleclick_event(MouseEvent& event)
     if (!model())
         return;
     if (event.button() == MouseButton::Left) {
-        if (!selection().is_empty())
-            activate_or_edit_selected();
+        if (is_editable() && edit_triggers() & EditTrigger::DoubleClicked)
+            begin_editing(cursor_index());
+        else
+            activate(cursor_index());
     }
 }