Add support for custom gallery navigation controls

[postmastersteve]

Galleries currently use Font Awesome icons for navigation arrows, fullscreen/collapse, play/pause, etc. Add support for:

• uploaded image
• tile design
• custom text (e.g. "prev" and "next") styled by font styles
• SVG design

Designers would like greater control over the appearance of gallery styles.

[postmastersteve]

+1 Matt Hudson ticket 187010