Support rule / action / event / condition categories in addition to groups

While groups are nice to have some simple grouping in select lists, that's not really sufficient yet. It would be nice to filter Rules by the elements used in there, e.g. so you can say show me all Rules related to "commerce payment", "e-mail", or "twitter".

I've been thinking whether we could simply tag the rule configurations with groups, but for one we have the translation problem (groups are only provided in the current translation), and the problem of mixing auto-generated stuff with user-controlled stuff.

So instead, the idea is to add a new "category" system, which basically replaces the previous groups - (or moves them a level down). So for the start, we can keep the UI as it is now, make sure existing groups and categories live well side-by-side and support filtering rules by what is already a category.

Thus, filtering options would become "Categories + Tags (+ optionally others, e.g. events).

In a next step, we could leverage it to do: #2052311: Improve action/condition/event selection screens
Also, in addition to having translation in-dependent category strings for search, we can start to allow modules to add (color-neutral) icons to a category (+ generating default icons for others). Then, fluxkraft and other UIs can start leveraging them, while Rules itself would not make use of it for now. If the icons work out well in other UIs, we could start using them in Rules itself as well.

So, this could work out like that:
- Add new category system for rules, which basically replaces the current "groups". Shows as "Category" in the UI and you can filter by Category + Tags - see #2052321: Improve filtering/searching for Rules.
- Categories are searchable, optionally have icons only
- For keeping BC, we support mapping groups to categories (creates 1:1 relationship). So contribs can keep using "group" for now and just assign the group to a category.
- Categories would be "fluxtwitter", "commerce", "commerce payment", "rules_mail", ... - and would have to be module prefixed (at least the machine name).
- Rules could probably need a list of useful pre-defined categories (mail, ... )
- Then, we could continue to support grouping inside categories: e.g. commerce payment could have groups like "paypal", "paymill", .. or "facebook" could have groups like "posts", "messages", .. (Would be only possible if used without group-cateogry-BC-mode").