Sidebar support

[Juraj-Masiar]

Modern browsers now supports sidebar for extensions (not just Firefox).
Although, it seems that Chrome uses separate API:
https://developer.chrome.com/docs/extensions/reference/api/sidePanel

Firefox API:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/user_interface/Sidebars
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sidebarAction