tabcounter: Optionality, styling, documentation (MR 62)

This commit adds an option to disable the tab-counter feature:
mcf.tabcounter.disable
Also, adding an option to 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 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