Use a smaller arrow text character as menu item separator.

The recent font stack changes result in our arrow separator (UI_MENU_ARROW_SEP) looking much larger than before. The following shows what it used to look like on the extreme left. To the right of that is how it looks now in current master using the same character. Next is one alternative character, while the last is the result of this patch:

Why the change and which version of U+25B6 is actually correct?

We used to use a single font that we assembled on our own, starting with one font but then adding to it from other fonts. This means that not only was our font old but it had internal inconsistencies. The character used (U+25B6) is meant to be a large right-facing solid triangle. The alternatives shown are meant to be "medium" and "small" versions of that. Our old one was simply smaller than it was supposed to be. This probably happened because we were trying to be economical with what characters we included because we had an approaching maximum. However now no longer have a limit and have full coverage of the symbols and arrows codepoint ranges.

Note that this patch uses the smallest of the arrows (U+25B8) to make it look as close as possible to the old font. However, without that consideration I would personally use the "medium" version (U+2BC8), shown above second from right.