[FEATURE] Make it more convenient to use VaadinIcons in @Menu

[MarcinVaadin]

Describe your motivation

When working with @Menu and wanted to set icon, it was natural for me to use VaadinIcon, but there is no simple way to do that.

I would expect something like:

@Menu(title = "Hello", order = 1, icon = VaadinIcon.CIRCLE.toIconString())

Currently I must use string value:

@Menu(title = "Hello", order = 1, icon = "vaadin:circle")

Desired solution

Would be handy if VaadinIcon enum had .toIconString() which returns vaadin: -name- or similar.

[Legioth]

What's needed instead is that the icon set needs to be expose string constants with the URIs to use. It cannot be a method call but it needs to be an actual static final field. It could then be like @Menu(title = "Hello", order = 1, icon = VaadinIconUrl.CIRCLE).

I'm moving the issue to the repo where VaadinIcon is defined.

https://github.com/parttio/line-awesome/pull/6/files shows an example of how this was done for Line Awesome