introduce search icon
    description file downloaded after validation, not before
    icons updated when engine added
    add opensearch xml description files
    sync desktop file => fix add webshortcut
    use db_opensearch.json file to track opensearch engines
    create an opensearch description only when the engine support suggestions
    remove opensearch description when an engine is deleted in the webshortcut setting dialog
    popup: disable ok button when webshortcut is already used

Lionel Chauvin is the man having done this big, big, big work!!
I'm just merging ;)