Add a chevron to distinguish editor menus with submenus

[peppy]

Closes #27916.

Before	After

[LittleLilyBun]

I'd hate to be a bother, but this doesn't seem to apply to right-click menus in the skin editor, is this intentional?

[Joehuu]

Shouldn't be. The chevron was added very locally (specific to the top editor menus). Probably for just less effort.

Ideally, this should be done in DrawableOsuMenuItem or even framework-side (as a default) as it is a crucial context menu UX.

[peppy]

Ideally, this should be done in DrawableOsuMenuItem or even framework-side (as a default) as it is a crucial context menu UX.

That comes with further considerations (doing it there requires blocking it from being displayed in other cases, at very least). I'm not against making it work there though – let me know if you're taking that on.

[Joehuu]

I'll try tackling that in DrawableOsuMenuItem for now.