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. Not sure if it’s the best name.
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 custom `cid` prop, it failed with backbone view cid set.

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.