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.
Merge pull request #357 from toebu/context-dependant-display

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