Handle dynamic updates to mode property

[ebidel]

From http://stackoverflow.com/questions/25585929/panel-setattributemode-waterfall/25586138

One example is that core-header-panel adds a tall class to a child with the class core-header:

waterfall-tall

If mode is updated later on to mode = 'waterfall', the tall class sticks around. core-header-panel should do this cleanup.

[gertcuykens]

Would be great if you make it small again using a slide effect when doing the cleanup :) The same effect as when you scroll down.

[frankiefu]

fixed by 29650b6