Revisit the use of arrows in menus/popovers

[acelaya]

We have been traditionally adding a triangle in popovers and menus, pointing to the element that opened them.

Without rounded corners these were frequently aligned to the very end of one of the element's sides.

With rounded corners this needs to be adjusted, causing some side effects and looking odd in some cases, because it either no longer points to the right place, or it requires the menu/popover to be relocated, causing scrolls to appear.

[robertknight]

To clarify the issue here, since we recently rounded many UI elements, including menus, the up-pointing arrow situated near the rounded corner on the right now looks a bit out of place, as you have a rounded curve that transitions into the side of a straight triangle:

[acelaya]

#5942 removed arrows in menus, but we still need to decide what to do with popovers.