[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/settings.yml

@@ -280,6 +280,7 @@ plugins:
 categories_as_tabs:
   general:
   images:
+  # stock_images:
   videos:
   news:
   map:
@@ -304,7 +305,6 @@ engines:
   - name: 500px
     engine: 500px
     shortcut: "500"
-    disabled: true
     timeout: 5
 
   - name: 9gag
@@ -920,7 +920,6 @@ engines:
     engine: www1x
     shortcut: 1x
     timeout: 3.0
-    disabled: true
 
   - name: fastbot
     engine: xpath
@@ -1005,7 +1004,6 @@ engines:
     disabled: true
 
   - name: flickr
-    categories: images
     shortcut: fl
     engine: flickr_noapi
 
@@ -1875,7 +1873,6 @@ engines:
   - name: picjumbo
     engine: picjumbo
     shortcut: pj
-    disabled: true
 
   - name: pinterest
     engine: pinterest
@@ -1916,7 +1913,7 @@ engines:
   - name: pixabay images
     engine: pixabay
     pixabay_type: images
-    categories: images
+    categories: stock_images
     shortcut: pixi
     disabled: true
 
@@ -2173,7 +2170,6 @@ engines:
   - name: shopify stock
     engine: shopify_stock
     shortcut: shs
-    disabled: true
 
   - name: sogou
     engine: sogou
@@ -2208,7 +2204,6 @@ engines:
   - name: stocksnap
     engine: stocksnap
     shortcut: sto
-    disabled: true
 
   - name: askubuntu
     engine: stackexchange