ANG-2680: Allow for multiple, consecutive spaces in caption of GridListImageItem.

Issue
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Multiple, consecutive spaces are condensed into one space for the caption of a `GridListImageItem`.

Fix
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We set the `white-space` property value to `pre` for the caption.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>