[enh] Initial Petalsearch Images support

Upstream example query:

  https://petalsearch.com/search?query=test&channel=image&ps=50&pn=1&region=de-de&ss_mode=off&ss_type=normal

Depending on locale it will internally use some/all results from other
engines. See:

  https://seirdy.one/posts/2021/03/10/search-engines-with-own-indexes/#general-indexing-search-engines

searx/settings.yml

@@ -1720,6 +1720,12 @@ engines:
       require_api_key: false
       results: HTML
 
+  - name: petalsearch images
+    engine: petal_images
+    shortcut: ptsi
+    disabled: true
+    timeout: 3.0
+
   - name: petalsearch news
     shortcut: ptsn
     categories: news