Somewhat support vertical tabs

This MR adds improves the usability when using Firefox's built-in vertical tabs feature.

Since we don't hide the vertical tabs selector in about:preferences, we should IMHO somewhat support them.

This MR changes the following:

• the sidebar can still be toggled when at < 700px width,
• alltabs-button (tab counter) is hidden at < 700px width,
• tab counter is disabled with vertical tabs at every width, as it would always show zero, which is never useful.

What it does not do:

- set the sidebar to the useful "hidden by default" state, as an attempt to do so led to having to set it to hide on every launch (and thus make matters only worse)

While I wish that we could make it fully useful out of the box (meaning, sidebar hidden by default in portrait, where things are narrow enough anyway), this already improves things enough to be mergeable. I've used it all week with the sidebar hidden and moved to the right side.

I've found a way to hide the sidebar by adding some CSS: !109 (305ebf77)

Here's what it now looks like:

and with tab-bar shown:

And in landscape:

... with tab-bar shown:

Here in landscape, the sidebar is not hidden by CSS, but it can be hidden, too: via the gear icon, the sidebar can be hidden, annoyingly, the menu needs to be opened again and the gear icon tapped, to leave customization mode.

Tested with everything from ESR 140.* to Nightly 150.