Adding swimlanes for optical logic encapsulation

Just noticed that this is the wrong place here and we need to move it to bpmn_io.

@mysdiir you can easily give this a try by deleting the following 2 lines from the bpmn_io.css file:

72: .djs-palette-entries .bpmn-icon-participant,
79: .djs-context-pad .bpmn-icon-participant,

Visually, that should already be all that's required. But we need to test of saving and executing models, where plugins are contained in such "participants" will still work as expected.

Ah, just tested a bit and it's not that simple because the XML structure changes completely. Each "participant" is treated like a model of its own, i.e. pretty much as if multiple ECA models were contained in a single one. That requires us to re-architect the \Drupal\eca_modeller_bpmn\ModellerBpmnBase class.