[Feature] Middle click new tab button to open new child tab

[SlugFiller]

From what I can tell, the only way to open a new child tab, currently, is to right click a tab, and select "Open a new tab as a child". A useful UI feature would be to allow accessing that option by simply middle clicking on the "+" button at the bottom of the tab bar. Or perhaps some other gesture like ctrl+click or shift+click. A gesture for opening a sibling might also be useful.

If there's some other way to quick-access this option that I've missed, let me know.