Fixed EXT-6597(normal) - Clicking the maturity rating icons in the navbar need...

Fixed EXT-6597(normal) - Clicking the maturity rating icons in the navbar need tooltips and clicking should link to a help article.

Added LLButton::setImagePressed(), there was no way to set pressed image at run-time. The only place where pressed image is initialized was LLButton constructor.

Replaced Maturity icon with button to simplify handling clicks. Icon does not provide click callback and we would need to handle mouse up/down events and track clicked widget.

Added icon for Moderate regions.

Did not find maturity help topic, added a "TODO".

Reviewed by Sergey Litovchuk - https://codereview.productengine.com/secondlife/r/243/

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branch : product-engine