[ENH] Add ability to specify not only class names, but also ids to newly created widgets

If on needs to manipulate the style or behavior of the widget from the js side, one has to use a class selector, which is suboptimal when having more than one widget of the same kind in the page.

Add e.g. add_id() method just below https://github.com/jupyter-widgets/ipywidgets/blob/master/ipywidgets/widgets/domwidget.py#L20

[rheiland]

Is there a (easy) way to make a Label boldfont or in color? I know we can make it italicized, e.g.: Label(value=r'\(This\ is\ a\ description\)'), but that seems a bit cumbersome. And how would one allow for “<” and “>” symbols? E.g., how can one display Label(‘A < B and C > D’), preferably in a color, say, red?

[b3m2a1]

Following up on https://github.com/jupyterlab/jupyterlab/issues/12252, the (lack of) ability to add names/ids to my HTML elements is becoming a huge hassle since I need to litter my interface with <a name="..."></a> tags so I can select against them in what is quickly becoming a shadow DOM…as JupyterLab decided to nerf any JS communication it would be nice to at least provide a way to add this little bit of help to the HTML the widgets create