Software tests and code base modernizations / refactoring

Alongside refactoring tasks like #24 (closed) and #60, the repository may start including some software tests when and where applicable. It is not an easy task with Gtk and ROS as dependencies, but the future may bring some changes to those subsystems, for example detouring from Gtk by using web technologies, e.g. #85.

Along the lines, we may also think about applying some more cleanups and refactoring to the code base, or fragments thereof, in order to shape it into the direction to be better approachable from other environments than Gtk and ROS.