Fix plugin menu icon background on hover

[msujew]

What it does

Closes #12826

Replaces the css rule on the icon element with a ::before element that emulates the same behavior. That leaves the background attribute on the icon element free to use ourselves.

How to test

Follow the instructions in #12826 to ensure that the behavior has been fixed.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[kittaakos]

Follow the instructions in #12826

I have verified (ed12938) the steps with the browser example. The fix worked. Thank you!

Before:

hover_menu__before.mp4

After:

hover_menu__after.mp4

The electron example does not yet show the icon.

[vince-fugnitto]

LGTM 👍 I can confirm the icon display persists when hovering