fix: do not share the Expr for text_color and background_color

[maartenbreddels]

Every TextRenderer shares the same Expr (by default). This means that if I create two text renderers, and change the expression on one, it also changes the expression in the other TextRenderer.e.g.

tr1 = TextRenderer()
tr2 = TextRenderer()
tr1.text_color.value = "'red'"
# now tr2.text_color.value is also red, bad UX I think

I think this should behave analogous to say a .layout widget, where they are not shared.

To be honest, this is not the original reason to create this PR. In solara, we cannot have widgets created at import time, since each user would share the same widget, which breaks ipydatagrid in solara.

I hope one of the two reasons is compelling enough :)

[maartenbreddels]

Originally introduced in #35 by @martinRenou

[martinRenou]

Thanks!

[martinRenou]

The commit I made from the Github interface has not been signed off, sorry for that! You will probably need to edit that commit (maybe you can also squash all commits into a single signed off one)

[maartenbreddels]

ok, squashed them, but needs you approval again :)

[maartenbreddels]

Looks like this collides with this https://github.com/jupyter-widgets/ipywidgets/pull/3358/files
I think we should open a separate PR and fix main/master first. What do you think @martinRenou ?

[martinRenou]

It's odd that we did not hit that before. PRs were green a couple of weeks ago. Thanks for the pointer! I agree we should probably fix master first.

[martinRenou]

Thanks!