[FEATURE] make it easier to integrate widgets with css frameworks

[ClaudiuCreanga]

Is your feature request related to a problem? Please describe.
It's hard to integrate bokeh with a css framework where you work by applying classes to html elements. When inserting a button with bokeh you get to chose from 4 types and there is an automatic default with colors.

 Button(
            label="Toggle",
            button_type="primary",
            css_classes=["btn", "waves-effect", "waves-teal"],
        )

The issue with this is that your bokeh styles are overwriting some of the framework styles and I have to go manually and add some other styles.

Describe the solution you'd like
I would like a fifth button type, call it empty, that has no styles attached to it, so when I use css_classes, those are the only styles applied to it. Also, I see that the classes are added to the parent div, I would like to also be able to target the button element with classes.

[bryevdv]

@mattpap do you have any thoughts on this proposal?