Add possibility to set an icon for a submenu. Current MenuModelRegistry.registerSubmenu only accept a label.
For example:

I'd like the Add submenu to have a + green icon.
@Hanksha I don't think active committers will work on it any time soon. An idea will be to add another argument to registerSubmenu, something like SubmenuOptions, which should be optional and it can have different properties in it. Feel free to work on it and send a PR.
@akosyakov Alright, I will do when I get some free time.
Got something working.

