LibGUI: Start working on GTableView inline editing.

This is pretty shaky still, but the basic idea is that you subclass GModel
and return true for editable indices. The table view also needs to have its
editable flag set.

Applications/VisualBuilder/VBPropertiesWindow.cpp

@@ -19,6 +19,7 @@ VBPropertiesWindow::VBPropertiesWindow()
     m_text_box->set_preferred_size({ 0, 21 });
 
     m_table_view = new GTableView(widget);
+    m_table_view->set_editable(true);
 }
 
 VBPropertiesWindow::~VBPropertiesWindow()