tabcounter: Optionality, styling, documentation

This Merge Requests adds a tabcounter feature.

Screenshot

It can be disabled optional via about:config.

Credit goes to:

user0/Emma for her great work to make this happen!

... and the reddit users mentioned in the source file.

/* Source from reddit users /u/BatDogOnBatMobile, /u/moko1960, and /u/It_Was_The_Other_Guy
 * https://old.reddit.com/r/FirefoxCSS/comments/s4wsww/
 * https://old.reddit.com/r/FirefoxCSS/comments/yb8tr9/ */

New options: To disable the tab-counter feature: mcf.tabcounter.disable Tto also count tabs in the tab menu with mcf.tabcounter.countmenu

We also opt to not show the tabcounter in convergent mode at widths over 900px (beyond the Fairphone 5 at 300% scaling in landscape mode).

Styling:

• make sure the enabled tab-counter is never rotated
• adding a rounded border to make it look somewhat native

Documentation: Mention and document disabling the feature in README.md (we should link new screenshots merging this).

approved this merge request

Awesome, thanks a lot Peter, Emma and folks from r/FirefoxCSS!

added 2 commits

• 77960f89 - tabcounter.css: Import 12_tab_manager_button.css from Emma's fenix branch (MR 62)
• 30da7374 - tabcounter: Optionality, styling, documentation (MR 62)

Compare with previous version

merged