Popover: Default to "bottom right".

[jasmussen]

It seems that if a position is not defined for a popover menu, it opens upwards from the center:

This causes a great deal of inconsistency, as can be seen in the GIF above, with some menus opening in a way that seems intentional, and others opening in a seemingly arbitrary direction.

Should we add some defaults to the popover opening method, so it defaults to opening bottom right (with an extra rule for LTR)? That way we'd have some consistency by default, yet individual implementations can still override and customize.

CC: @ellatrix