Modularize to a library and an ui component

[ttoenige]

No description provided.

[marc-hanheide]

I think this is a worth-while effort. In fact, @Jailander was working on something similar I believe in our own branch https://github.com/LCAS/gazetool, though he hasn't committed anything there yet. I think we should base our own effort to ROS-ify gazetool on this stand-alone library. We'd like to join efforts with @ttoenige on this. But @lschilli should have his say and comment on design recommendations here.

I recommend to refactor this so it has a QT-free core library, that facilitates building wrappers around it, with one being the QT-based GUI and one be the ROS component we'd be contributing. We discussed this also in #1 at length. Maybe @Jailander could comment on issues he had and make recommendations as well.

In any case 👍 for this effort and this PR to be merged 😄