Customise viewer tools

[ivmartel]

The user must be able to customise the default viewer tools (scroll, zoom, contrast...) from the plugin settings page. This default list must be over-ridable at a blog post level if needed.

This will imply modifying the wordpress code and the viewer javascript ([dwv-simplistic #88](see ivmartel/dwv-simplistic#88)).

doc: https://developer.wordpress.org/plugins/settings/custom-settings-page/