[IMP] website.snippet: create automatically the thumbnail; data-option became data-js; add data-eval method; rename check_class into toggle_class
[FIX] website.snippet: the background option is not readily usable by the designer
Merge pull request #2050 from odoo-dev/master-theme-review-chm

[IMP] website.snippet: create automatically the thumbnail; fix background
[IMP] qweb debug mode restricted to --dev mode

Also moved the set_trace call in top of render_node() function so it
makes more sense.
[IMP]Export Wizard: Improve code to not remove fields from "Fields to Export"on Export all data and remove noncompatible field on selecting Import Compatible Option