Add getRegion to Layout

Fixes #1127

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**Changes**
- Added `getRegion` method to `Layout` (from [Application](https://github.com/marionettejs/backbone.marionette/blob/v1.7.4/src/marionette.application.js#L69-L74))
- Added  `Layout.getRegion` test to `layout.spec`
- Added `Application.getRegion` test to `region.spec` (probably doesn't belong there but thats where the rest of the Application region tests were... :kanye_shrug:)

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Quick question: Since Application and Layout share a lot of RegionManager-related methods, should they be abstracted somewhere to be reused?

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Let me know if this pull request needs anything else. Thanks!