Change handling of exercises

At the moment exercises are created with:

# \exercise{label}
## Exercise Title

This has some drawbacks:

1. The frame has a non descriptive title like "Exercise 8.1". This title is used for example in the schedule. I addition, there are no exercise number in the curriculum, only the exercise title.
2. The handling of this title by latex is very fragile. For example, it is very tricky to make a good hypertarget for it resulting in a hack. Sometimes the linking does not work properly.
3. It would be nice to add the mode and duration of an exercise. After the title there may not be enough space for it. And the frame title is not the right place for it. Exchanging frame title and exercise title and putting the information after "Exercise 8.1" would be a solution.
4. If we mark an exercise as optional by surrounding with brackets, this will not be visible in the note page (because of 2.)

I suggest a new implementation:

# Exercise Title \label{ex:label}

## \exercise{mode}{duration}