Make the iowait example plugin a controller too
Make the iowait example plugin a controller too

It exposes a button that allows switching between showing an iowait
statistics and an idle statistics. When the button is pressed it
should be replaced with other button. The button is shown in the host
node.

This is a rather nasty case as it shows several problems:

- Button control races
  - The way the NodeControl currently works creates races between
    plugins adding buttons to the same node. This is because
    NodeControls are not really merged, but rather one of the two are
    chosen based on a NodeControls' timestamps, so the older one is
    thrown away entirely. In the end GUI can switch randomly between
    showing controls from one plugin or from another.

- Showing outdated statistics
  - When pressing the button to switch to show the other statistics,
    the old ones are still shown for several seconds.

- Slowness of the updates in GUI
  - Pressing the button yields no immediate reaction. Changes happen
    after several seconds. Probably related to the previous point.

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