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

src/applications/files/iconset/PhabricatorIconSet.php

@@ -3,6 +3,8 @@
 abstract class PhabricatorIconSet
   extends Phobject {
 
+  abstract protected function newIcons();
+
   final public function getIconSetKey() {
     return $this->getPhobjectClassConstant('ICONSETKEY');
   }