BECConnector method to handle `QThreads` for users

Problem

While writing the documentation on how to develop a widget, a button has been develop that starts a scan. This call can be made blocking, which opened the question how to facilitate the easy handling of QThreads in the BEC Widgets framework.

Proposed solution

Add functionality to for instance the BECConnector class to give users the possibility to run a user-defined function in a QThread, and specify actions to be executed upon interrupting or finishing the task.