Add callback function for Zikula.UI.Panels activation

[Guite]

It would be nice if one could inject JavaScript code which is executed after the activation of a panel has been completed.
Concrete requirement: redraw a Google map which just became visible.
Without redrawing the map explicitly it will look broken as soon as it isn't visible at once after the page load.

See also Guite/MostGenerator@92e5e73

@jusuff

[Guite]

@gfr

[ghost]

I think we are dropping zikula.ui

[Guite]

Closed as not necessary anymore. Bootstrap offers the collapse component which provides an event named shown.bs.collapse which will be implemented as part of Guite/MostGenerator#238