Describe the bug
When adding a css class to an element with a label, the css attribute of the label DOM element is not updated with the new class.

For instance, after calling

bpmnVisualization.bpmnElementsRegistry.addCssClasses('task_3', ''additional-info']);

To Reproduce
bpmn-visualization: 0.20.0
See process-analytics/bpmn-visualization-examples#264

Screenshots

Add css class names, labels are not styled.
After doing a zoom or a pan, labels are styled.

Additional context
The same behavior occurs when calling toggleCssClasses or removeCssClasses.

When updating the mxGraph model, we only update the shape of the bpmn element but not the shape of the label, and we only redraw the shape of the bpmn element.
See https://github.com/process-analytics/bpmn-visualization-js/blob/v0.20.0/src/component/mxgraph/MxGraphCellUpdater.ts#L43
On zoom or panning, the whole model is computed and redrawn, so the label shape contains the right css classes.
So, in MxGraphCellUpdater, we should also set state.text.style. See also https://github.com/jgraph/mxgraph/blob/v4.1.1/javascript/src/js/view/mxCellRenderer.js#L427

Notice that for now, we don't have integration tests that check the DOM elements produced for labels. We should add some to check that the right css classnames.
For the record, specific bpmn-label class is applied to label DOM elements #956 on 2020-12-18.