Repaint request implicit through rootPainter

List of improvements:

* tuikit.Init() does not return a repaint channel anymore. A repaint
  should be requested through NeedPaint() in the root window (aka the
  rootPainter).

* Setting the rootPainter will request a repaint right away since there
  isn't any way for a client to request it explicitly.

* Setting the rootPainter will also register its repaint callback to
  request a repaint of the whole screen. Clients don't need to register
  it explicitly anymore.

* Various code refactorings in the examples, reducing their complexity.