book: Add chapter about paintables, event controllers / gestures, layout managers

[sdroege]

All relevant when doing your own widgets. The first two especially if they're not just container widgets, the latter specially for container widgets.

[sdroege]

• make an example of a silly painting widget where you can draw rectangles and they're rendered as rectangles and you can abort the current rectangle by pressing esc (-> event controller, gdk snapshot) and allowing to insert/position pictures (-> paintables, event controllers).
• making a simple box container widget (layout manager)

[Hofer-Julian]

Outline:

• custom widget painting
• paintable
• layout manager
• gesture