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.

[lbwexler]

Raising the priority of this. The ContextMenu API was actually substantially changed in Blueprint 4.0, and this is actually the main blocker to upgrading to Blueprint 4.0.

The new api does not provide an obvious way to respond to the actual mouse event, and dynamically construct the menu at that time. Rather than invest more time and contortions into targeting the new blueprint context menu api we should just promote some verison of the mobile menu as a desktop (or cross-platform) native Hoist menu component.

[amcclain]

https://github.com/szhsin/react-menu