Adding a new functionality that allows the context (i.e. css classes)

of an element to be changed easily. Using this to implement the
'remaining todos' and 'complete todos' views. Reverting fix from
69e5778 that is now not needed anymore since task deletion works as
it used to.

I had to implement something myself, since there was no bootstrap functionality that would do something like that and since derby refLists and filters don't play nice together.

This replaces PR #277

The new changeContext function will also be handy when implementing #208.