[FEATURE] Make hover tooltip arrow style configurable

[allenfrostline]

It's actually a very simple one, see image below:

Instead of the current style, won't it be better looking if we have the grey triangular head colored in white (with some transparency, too) just like the rest of the hovering tooltip? I've been looking for ways to change that color using css etc. but it's definitely better if the coloring can be done from an interface provided by the package.

[bryevdv]

There is no accounting for preferences @allenfrostline I actually would not like the "arrow" to match and in fact would rather it be much darker. But we can definitely consider making it more directly configurable.

[arefeena]

Hey, I am new to the project and I would like to take a crack at this.

[bryevdv]

@arefeena We would love to have your contribution. New contributors arrive with a wide variety of experience we can't predict, so please let us know where we can help provide guidance once you start to take a look.

[arefeena]

UPDATE: opened PR #10750 so we can discuss specifics there

@bryevdv looks like I got it working on my branch: arefeena/9378_tooltip_arrow_styling

I ran the examples/app/gapminder demo, replacing the existing HoverTool creation (L51) with the following (black could be replaced with any CSS colour-like string e.g. #000000 or rgb(0, 0, 0) etc.):

plot.add_tools(HoverTool(tooltips="@Country", tooltip_arrow_color='black', point_policy='follow_mouse'))

It looks like this:

Are there other attributes besides the arrow colour that should be configurable?