Fix call to non-existing newIcons() in PhabricatorIconSet

Summary:
PhabricatorIconSet::getIcons() calls $this->newIcons() which is undefined.

As all six child classes which extend PhabricatorIconSet implement newIcons(), declare an abstract function in their parent class.

Test Plan: Carefully read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25647