[FEATURE] PHP Piwik tracker integration

The PHP API of Piwik is now used to build an image tag that will be
used if JavaScript is not available.

See: http://piwik.org/docs/tracking-api/

Resolves: #11246
[FEATURE] Allow global configuration override

This patch will override the settings from config.tx_piwik with
those provided to the Footer.php user function.

Addtionally the obsolete $tc property is removed and the PHPDoc of
the contentPostProc_output() is improved.

Resolves: #56081
[BUGFIX] Do not cache the tracking code

To prevent problems with different environment conditions (e.g. Backend
user logged in or not) the USER function for rendering the footer
was changed to USER_INT.

Resolves: #51289
[FEATURE] Configurable custom variables

It is now possible to submit custom variables to the Piwik tracker
by using the "customVariables." config key.

Resolves: #34367