Example 74: Add two kinds of properties to OrkivRoot.

These will facilitate calculations about whether to add a new chat window beside the buddy list or
or to replace it.
Example 75: Set the mode based on the width property.

Kivy's dynamic property binding means that the mode will always be set as soon as a resize event occurs.
Example 76: When adding a chat window, decide whether or not to keep buddy list visible.
Example 77: When window is resized while chat_window is visible, determine whether buddy list needs to be shown or hidden.