Add search operation tracking

[aeirola]

Should we straightforward to just call the underlying methods. Might need to check what would be a good name for the number of search results parameter. Maybe align with whatever the Piwik web interface uses?

trackSearch(keyword: string, category: string, resultCount?: number)

See:

• https://developers.piwik.pro/en/latest/tracker/tracker_object.html#trackSiteSearch
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_Android.html#tracking-search-operations
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_iOS.html#tracking-search-operations