Add proguard config for Creatable

[kurtisnelson]

If a scope is only instantiated via the new Creatable method, proguard will strip the Impl with the current config. As a temporary workaround, all subclasses can be kept via:

-keep class * implements motif.Creatable {
    <init>(...);
}

However long-term the solution is to use -if to pair the impl with the kept state of the interface.

[andreasnomikos]

wait didn't we add a test for this exact thing ?