[Menu] Add Menu child components as static properties of the Menu component

[rosscourt]

Not related to an issue.

Checklist

• Enable CircleCI for your fork
• Update documentation

Changes proposed in this pull request:

Added static properties to the Menu component to allow easier importing of it's child components MenuDivider and MenuItem. Extending an existing pattern found on the NavBar component.

Allow imports such as:

import { ..., Menu, MenuDivider, MenuItem, ... } from '@blueprintjs/core';

to be reduced to:

import { ..., Menu, ... } from '@blueprintjs/core';

Where usage of MenuDivider would be changed to Menu.Divider etc...

I've also aligned the reference to this pattern on the Menu component's documentation page to match the NavBar documentation.

[rosscourt]

Add Menu child components as static properties of the Menu component

Preview: documentation | landing | table

[giladgray]

👏 👏 very nice @rosscourt! totally down for this.

[giladgray]

@rosscourt just so you're aware, this will ship in the next 3.0 beta release later this week.