I wonder if anybody else needs a time slider? I want to present a few
really simple blocks (rectangles), but they change over time and I want the
user to see the current time as well as give the opportunity to slide to a
certain position - you know, the YouTube movie kind.

There is a list of events each with a time and those mean to add, remove or
fade a simple rectangle.

Everything else looks quite fantastic, I'm really looking forward to use it
for visualizations.

Sorry, but I couldn't find how to mark it "Feature Request".

This is the sort of thing we rely on libraries for--check out controlp5 or
one of the other GUI libraries on the libraries page:
http://processing.org/reference/libraries/

The issue is that to properly handle components, you quickly need a lot of
infrastructure to handle them, so it's best left to a complete interface
solution.