Implements the new general design (thanks to Clement's and Jule's mockup)
Allows the display or the hiding of non-compatible plugins
Add advanced filters in the page (harmonization with the advanced filters in the user manager)
Add the display of a screenshot, the rating and the tags for each plugins
Add Piwigo's Certification levels for each plugins
Add the file admin/themes/default/css/components/general.css to save css compnents that will be used in diffenrent places in the admin
