SkelVanilla : a11y and usuability

A template based on SkelVanilla adding some a11y improvements : usage of aria-labelled-by or aria-desscribed-by with dynamic help and user help, keybord navigation tested and fixed, allow usage of title for question text … etc …

This theme are used on Demo website, and you can test a complete LimeSurvey sample survey.

Include bootswatch theme for variations and 4 different variations for each theme; allow more control on color and question presentation using bootstrap system.