Since we're typically interrested in *where* (on the progress-bar) the
user clicked, we need a way to pass the clicked position to the
handler.
Normally, the on-click handler is expanded when a particle
instantiates its exposable. At this point, we (obviously) don't have
the click position.
This is solved by expanding the handler a second time, when the bar is
clicked.
Thus, the user can use the "{where}" tag in the click handler. "where"
will be expanded to a percentage value (0-100).
