Make action types pluggable

Currently the list of accepted action types is static, and handled in /nuxeo-platform-webapp-base/src/main/resources/web/nuxeo.war/incl/action/generic_action_template.xhtml

This should be pluggable as widget types are => need to refactor templates, turning each specialized template into a widget template, and reference it in a widget type only available in a new category "jsf_action" ("jsf" is the default category).

Then the generic_action_template can change its code to call a nxl:widgetType tag instead.