Switch in IPE where new pane gets added, top or bottom

Opening issue which originated from #1690128: Add Content to Top of Region in IPE?

Problem/Motivation

When adding a new pane, it defaults to top of region for usability concerns. It would be nice if it could be made an option - either top or bottom. This is especially nice for left navigation menus that need to consistently be in one place (users with permission could possibly alter the UX of a site in one fell swoop).

merlinofchaos:

"In the future this probably needs to be switchable somewhere, because I also just committed a patch that makes the position of the control themable. So presumably something should also be able to switch where the pane gets added to match."