ENYO-36: Add the ability to specify controlsPerPage in a DataList.

Issue
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There does not exist a way to override the dynamically calculated `controlsPerPage` property for `enyo.DataList`.

Fix
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If a value for `controlsPerPage` is provided upon initialization, this value will be used and dynamic calculation will be bypassed.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>