Allow the user to "Pin" tabs

With #5787 merging soon, we'll be able to have compact tabs, the same size as "pinned" tabs in lots of browsers.

We should add a context menu option to let users pin tabs as well. When tabs are pinned, they'd move to the start of the list of tabs, to the left of the test of the tabs. We'd probably need to have different lists for pinned vs normal tabs.

Trickily, pinned tabs could only be reordered within the range of the pinned tabs - so it can't be [pinned, pinned, normal, pinned, normal] tab. That might be tricky to pull off.

A follow-up could be linked to #961 where the pinned tabs are re-started, but not the unpinned ones. I think this is kinda similar to how browsers handle pinned tabs. We'd need to reconcile this with #756 - If the user has tabs pinned, then do those re-open, and the startup configuration the user specifies? We'd want to have a comprehensive story before we start on either story.

Start by reviewing the compact-tab work in #5787 and the related startup discussions in #961 and #756. Trace the tab context-menu and tab-list entry points, then define how pinned and normal tabs are separated, reordered, and restored. Done means pinned tabs stay at the start, can be reordered only among themselves, and the startup behavior is reconciled with existing configuration.