Nonblocking

[sergiobuj]

This PR will run the algorithm on a background thread (resolves #3).

• This PR changes a lot of the code, since making the execution of the algorithm non-blocking required to move many tasks and functions around.
• The Worker was introduced to run the algorithm on the background. Workers will make it very easy to extend for future algorithms.
• This PR gives a better separation of plugin, algorithm dialog and algorithm execution to allow for a less painful addition of new algorithms.