LayoutManager
  VerticalLayout
  HorizontalLayout
  TileLayout


events:

  draw()
    - caller
  on_draw()
    - core handler, for overloading
  connect('draw', my_draw)
    - additional handler


Focus
-----

 * only one node in hierarchy can have focus
 * all parent containers have focus, so they can relay events to child
 * top container has always focus

 * grab_focus() on any node will clean old focus and set focus to this child
 * global shortcuts can be handled in keypress handler of top widget