Massive layout changes

1. Don’t show / hide views in layout. Closes gh-528.
2. Add View#keepElement (was needed for change 3). If it’s true, $el.remove() won’t be called. Instead, all events will be unbound from view.
3. Inherit Layout from View, bind it to body instead of document.
  Renamed Layout methods and properties:
	* `_registeredRegions to `globalRegions`
	* `registerRegion to `registerGlobalRegion`
	* `registerRegion to `registerGlobalRegions`
	* `unregisterRegion to `unregisterGlobalRegions`
	* `unregisterRegions to `unregisterGlobalRegions`

Removed layout events spec as it’s not needed because we have same view spec.

We had enough borrowed methods from View in Layout so just inheriting it will be cooler.