Offer omnibox (address bar) handler for switching the current tab to a different container

[ErichDonGubler]

Description

Extensions like Switch Container Plus allow a user to very conveniently switch containers for the currently viewed tab. It's pretty great!

However, other extensions cannot be not aware of Sidebery's source of truth WRT tab placement, indentation, etc. From Firefox's perspective, the old and new tabs between containers are separate, with nothing to link an old tab to a new tab to an unrelated extension (which is a good default!). This means that when using them—as a continued example with Switch Container Plus, typing co My Container—Sidebery orders the new tab at the end of the sidebar, by default, losing the old tab's placement.

Sidebery should add a handler for the omnibox (keyed using a bikesheddable keyword like sb) to allow users to quickly change the container of their tab while keeping the placement of their tabs.

[ErichDonGubler]

I've been test-driving #1992 myself to experiment with a solution for the last few months, and I've enjoyed it immensely. I'd encourage other folks to try it out.