Allow qtivi to use custom templates folder

Currently adding some custom templates, that are used during the build as part of a module (that will be installed under the Qt install prefix) fails to find those custom templates. the generator tries to look for those templates in the ivigenerator folder. So in that case we are forced to do a make install or copy the custom templates to the ivigenerator install path to be able to proceed.

We need a way to allow using such custom templates, maybe the same way that custom filters can be set using QFACE_FORMAT.