CustomWizard: Use object pool to find factories

Rename class type of CustomQmakeProjectWizard from "qt4project" to
"qmakeproject" and document that change in the manual and changes-3.1.0.

Also update all the in-tree xml files to that new class name.

Change-Id: I7a9ef914a429d68e7f37a2115c03fa81d7433398
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>