Support for Split View

[aecreations]

Split view is a new feature introduced in Firefox 146, but not visible by default (needs to be turned on via a pref, browser.tabs.splitView.enabled). WebExtension API support is being added starting in Firefox 149[1], with more APIs supposedly forthcoming[2].

If the "Hide and Replace" action was selected, Panic Button needs to be able to recreate the split views when restoring browser windows.

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[1] Firefox 149 release notes for developers: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/149
[2] Add extension API to create split views, or remove split views: https://bugzilla.mozilla.org/show_bug.cgi?id=2016928

[aecreations]

More info on the new Split View feature: https://www.howtogeek.com/firefox-is-testing-a-split-screen-mode-and-tab-notes/

[aecreations]

Confirmed on Firefox 149 that any existing split views are lost when restoring the browser window. Although the tabs are intact and placed side by side, they appear as separate, ordinary browser tabs.