Make compact (many in one row) pinned tabs configurable.

[dos1]

While pinned tabs being outside of scrolling container are awesome,
for old Tab Center-like configurations with autoshrinking sidebar
the presentation of many tabs in one row is actually an antifeature.

This PR makes it configurable, so custom userChrome.css configs like
in #103 or #118 that restore old behavior can work properly.

Also, the thumbnail reseting has been removed. This fixes a newly
introduced issue of unpinned tab not having its thumbnail updated.
While this could be fixed in other way, disabling thumbnail generation
for pinned tabs is not desired when compact pins are disabled anyway.

[eoger]

Give me some time to think about this.
I really really don't want to have to maintain 2 implementations of the same feature.

However, I'm aware that a number of users like you have (beautiful) CSS customizations that probably rely on the pinned tabs being displayed as rows.
If I left the pinned icon in the webextension files, would you be able to reproduce the "legacy" look with CSS only? We haven't made a lot of markup changes, so I believe it might be possible.
It would be a win-win: you get to keep your pretty CSS customization while it lifts the burden of maintaining on second implementation of me. What do you think?

[dos1]

I initially wanted to make a PR that just generalizes the styles to be applied by classes instead of IDs, so adding custom CSS for that would be easy, but then I thought "hey, it could be just a matter of toggling the class name", so added an option for it. Take notice of how this doesn't actually add many new styles - if compact pinned tabs are disabled, the styles for regular tabs are used (and if they're compact, then the styles are just overriden). The rest is just fine tuning or making the stylesheet more elegant. Thanks to that, you could for instance easily apply the same styles as for pinned tabs for every other tab as well just by toggling the class name ;)

[dos1]

@eoger: Just noticed the 723b42e commit. Looks great, thank you!

I can see some minor background color tweaks to be made there, but let me PR it separately :)

[nirfse]

Just came here to say "thanks" for the feature I was waiting for!