Remove close button on tabs unless hovered

**Is your feature request related to a problem?**

I don't like tabs always showing the close button, including the active tab. (Literally hate it lol.)

**The solution I'd like**

Expand [this section](https://github.com/black7375/Firefox-UI-Fix/wiki/Options#remove-close-button-at-hover) to include something like `Only show tab close button on hover`, so that no matter whether a tab is active or inactive, you can only see the close button by hovering over a tab.

**Screenshots or video**

**Alternatives I've considered**

[This](https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/tab_close_button_always_on_hover.css) works. ~after changing `userChrome.tab.close_button_at_hover` to `false`.~

**Additional context**

Start with the Options wiki section linked in the issue and compare the behavior of the referenced tab_close_button_always_on_hover.css. Locate the existing tab close-button option in the repository's SCSS and extend it so close buttons appear only when hovering either active or inactive tabs. Done means the option is documented and both tab states follow the hover-only behavior.

Written by the indexing model from the issue text.