Re-structure Plotter code

[zoccoler]

The code for the plotter is a bit too long and could be simplified to ease maintenance.

An idea to fix this is to put a basic plotter version in another repository and derive a class here, integrating it with some UI made with the Qt designer for example.

I released a version of such a plotter here: biaplotter. Hopefully it can be used for that purpose.

Some initial ongoing work related to this was started in #305

[jo-mueller]

https://github.com/BiAPoL/napari-clusters-plotter/tree/v0.9.0