Desktop Menu component(s) - replaces direct use of Blueprint

[amcclain]

This is a weird gap that came up in our discussion on PR #1800.

We provide Hoist-level support for menus in a few places:

• Desktop contextmenu package - https://github.com/xh/hoist-react/tree/develop/desktop/cmp/contextmenu
• Mobile menu package - https://github.com/xh/hoist-react/tree/develop/mobile/cmp/menu
• Desktop/Mobile AppMenuButton comp

But we don't provide a first-class way to setup e.g. a button on a toolbar that shows a menu - you need to use BP kit components.

We should review and reconcile the existing approaches and come up with a first iteration (however minimal) of a Hoist menu package for desktop.