Add download tracking

[aeirola]

Unsure if Android actually support tracking downloads that are not application installs. Would need to test how the SDK actually behaves.

See:

• https://developers.piwik.pro/en/latest/tracker/tracker_object.html#download-and-outlink-tracking
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_Android.html#tracking-downloads-and-app-installs
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_iOS.html#tracking-downloads