Can we give menuitems a default command?

[domfarolino]

When working on #1324, I noticed that lots of code examples had repetitive command="show-menu" attributes:

It's annoying to have to write this, and it feels unnecessarily repetitive and verbose. In reality, when you're using a <menuitem> with a commandfor attribute, you're probably using it to open another sub-menu. And when you're not, you'll need to specify the command anyways. So can we just make show-menu the "default command" for these elements? You can manually set command when you want to override this, but otherwise you won't have to provide it to get the normal sub-menu-invoking behavior, which seems pretty sensible to me.

Thoughts?

[lukewarlow]

Would we want the default to be show-menu or toggle-menu is one question?

We did specifically decide against implicit commands for buttons. But I think this is a case where the context might be scoped enough that it's clear.

I worry that people will wonder why a button pointed at a menulist still needs an explicit command tho.