Fix button being visible while widget is closed (in Firefox)

[raucao]

Since a recent new version of Firefox, some of widget's content elements are visible, while the widget is closed, due to a CSS selector being prioritized differently now. Here's what it looks like in Webmarks and Sharesome for example:

This adds an additional selector to prevent overriding attributes when the widget content area is hidden.

[DougReeder]

Do we have a policy to do merge commits, squash & merge, or rebase & merge?

[raucao]

I think it's just the same as for the main library: https://remotestoragejs.readthedocs.io/en/latest/contributing/github-flow.html