Add a way to report progress

[laeubi]

In IDEs it is common to show the user some kind of progress of the actual task.

It would be good to have such thing as well for the build API, the idea is that one can acquire a Progress object for some kind of executable object (e.g. a Mojo) and then report progress of the current executing mojo.

[laeubi]

I just noticed that there is even a better way:

Because a mojo execution can be observed the implementer can use this to automatically get the Mojo requesting progress and thus no passing of the executable object is required at all!