[mod] Startpage: reversed engineered & upgrade to data_type: traits_v1

One reason for the often seen CAPTCHA of the Startpage requests are the
incomplete requests SearXNG sends to startpage.com: this patch is a complete new
implementation of the ``request()`` function, reversed engineered from the
Startpage's search form.  The new implementation:

- use traits of data_type: traits_v1 and drop deprecated data_type: supported_languages
- adds time-range support
- adds save-search support
- fix searxng/searxng/issues 1884
- fix searxng/searxng/issues 1081 --> improvements to avoid CAPTCHA

In preparation for more categories (News, Images, Videos ..) from Startpage, the
variable ``startpage_categ`` was set up.  The default value is ``web`` and other
categories from Startpage are not yet implemented.

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