[mod] add new category 'stock_images' - a specialization of 'images' (#6703)

Engines like Pexels provide images that one would more likely expect in a
category named ``stock_images``.

By classifying engines like Pexels more precisely, we enable a more specific
handling of them, which also helps to avoid mixing them with the more general
image search on the internet. [1]

We leave these engines *enabled* by default, take them out of ``images`` and
move them into the specialization ``stock_images``.

This makes it possible, on the one hand, for the admin to configure a tab in the
UI, and on the other hand, the user can always select the group directly using
the search syntax ``!stock_images ...``.

What we need to keep in mind:

1. Technically speaking, there are no subcategories in the strict
   sense.  Organizationally, the *subcategories* are derived from the
   `categories_as_tabs`.

2. From an admin’s perspective, `categories_as_tabs` is the tool for
   structuring their content.

3. In the context of “enabled/disabled by default”: We should avoid having
   the admin have to activate many individual engines when they want to
   structure their content (UI tabs).

So if we leave an engine enabled by default and **move** it from `images` to a
new category, the admin only needs to add the new category to
`categories_as_tabs` to restructure their content.  If the new category is also
part of the translations (see `searx/searxng.msg` in this PR), this UI structure
would also be available internationally.

[1] https://github.com/searxng/searxng/pull/6690#issuecomment-5646291679

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/searxng.msg

@@ -31,6 +31,7 @@ CATEGORY_NAMES = {
     'MUSIC': 'music',
     'SOCIAL_MEDIA': 'social media',
     'IMAGES': 'images',
+    'STOCK_IMAGES': 'stock images',
     'VIDEOS': 'videos',
     'RADIO': 'radio',
     'TV': 'tv',