[Feature request] Add additional sidebar

[menganatha]

Would it be possible to add an additional, 2nd (or 3rd if we count the native one) sidebar?
Use case explanation:
Native sidebar is reserved for Sidebery/TsT
1st sidebar is reserved for some permanent app running in the background (e.g youtube)
2nd sidebar is used as a helper tab for the contents of the main tab, e.g for quick comparison or dragging things around. Basically treated as a supportive tiled/split tab.