[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