ENYO-2286: Add samples for core Enyo UI controls

Issue
--------
We need samples to show the basic UI objects as well as to serve as an onyx-free testbed for those controls.

Fix
----
An additional `UI` section has been added to `Enyo Core` in the `Sampler`, providing access to samples that span the core Enyo UI controls (several existing UI samples, mainly those concerning Repeaters and Lists, have been moved into this section as well).

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>