diagram description (#233)

1. I want to edit the content of a 'diagram description', but I don't know how to do that... some pointers please...

2. I tried to the context menu 'documentation' on the 'Diagram Description' but that forced an error in SIM.

Content call stack:

System.NullReferenceException: Object reference not set to an instance of an object.

at DockablePanel.Controls.DockablePanel.DockingManager.FocusPanel(DockingWrappedPanel panel)

at SoftwareIdeasModeler.MainForm.ShowDocumentation()

at SoftwareIdeasModeler.SimDiagramControl.documentationToolStripMenuItem_Click(Object sender, EventArgs e)

at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)

at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)

at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)

at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)

at System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met)

at System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met)

at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)

at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)

at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.ScrollableControl.WndProc(Message& m)

at System.Windows.Forms.ToolStrip.WndProc(Message& m)

at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)