Make Ministro connection code independent of the app store distribution

Currently, an app deployed with Ministro-bindings will use a specific distribution of Ministro with a hardcoded app store identifier.

In order to make it possible to deploy Qt apps which depend on other distributions of Ministro, this should be made customizable in the back-end. Whether we want to make this customizable per project is not yet determined, but the framework for doing so should be in place in case that becomes a requirement.