[feat] engines: add swisscows images engine

The implementation is basically a 1:1 port of the reverse engineered
swisscows JavaScript code. (it's been obfuscated, so I've restructured it
and made the variable names idiomatic instead of obfuscated var names like "a", "o", "i")

```js
/*
e: "/v5/images/search"
t: {
	itemsCount: "50"
	locale: "de-DE"
	offset: "50"
	query: "test"
	spellcheck: "true"
}
*/
// HASH library used: https://github.com/h2non/jshashes
function generateNonceAndSignature(queryParams, urlPath) {
  // urlPath = "/v5/images/search"
  // sort keys alphabetically and join to query string
  let queryStringSorted = '?' + U().stringify(queryParams, {
    arrayFormat: 'repeat',
    allowDots: !0
  }).split('&').map(e => {
    let[key, value] = e.split('=');
    return [key, decodeURIComponent(value)]
  }).sort((e, t) => e[0].localeCompare(t[0])).map(e => e.join('=')).join('&');

  function caesarShift(str, offset = 13) {
      const alphabet = 'abcdefghijklmnopqrstuvwxyz';
      let result = [];
      for (let a = 0; a < str.length; a++) {
        let c = str[a],
        alphabetIndex = alphabet.indexOf(c.toLowerCase());
        if ( - 1 !== alphabetIndex) {
          alphabetIndex += offset;
          while (alphabetIndex >= alphabet.length) alphabetIndex -= alphabet.length;
          c = c === c.toUpperCase() ? alphabet[alphabetIndex] : alphabet[alphabetIndex].toUpperCase()
        }
        result.push(c)
      }
      return result.join('')
    }
  const r = new (sha256Instance()).SHA256;
  const random = randomString(32);
  const randomShifted = caesarShift(random);
  let to_hash = [urlPath, queryStringSorted, randomShifted].join('');
  let signature = r.b64(to_hash);
  signature = signature.replace(/=/g, '').replace(/\+/g, '-').replace(/\//g, '_');
  return {
    nonce: random,
    signature: signature
  }
}

function randomString(length) {
  let t = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~',
  n = '';
  for (let r = 0; r < length; r++) n += t.charAt(Math.floor(Math.random() * t.length));
  return n
}
```

searx/settings.yml

@@ -2540,6 +2540,11 @@ engines:
     disabled: true
     inactive: true
 
+  - name: swisscows images
+    engine: swisscows_images
+    shortcut: swi
+    disabled: true
+
   - name: wordnik
     engine: wordnik
     shortcut: wnik