Implement cleaner code for backend configuration

...est/kotlin/de/justjanne/libquassel/client/EndToEndTest.kt

...n/de/justjanne/libquassel/protocol/features/FeatureSet.kt

...in/de/justjanne/libquassel/protocol/models/BackendInfo.kt

...janne/libquassel/protocol/models/BackendInfoSerializer.kt

.../justjanne/libquassel/protocol/models/HandshakeMessage.kt

...lin/de/justjanne/libquassel/protocol/models/SetupEntry.kt

...protocol/serializers/handshake/ClientInitAckSerializer.kt

...el/protocol/serializers/handshake/ClientInitSerializer.kt

...ain/kotlin/de/justjanne/libquassel/protocol/util/pairs.kt

...n/kotlin/de/justjanne/libquassel/protocol/util/triples.kt

.../de/justjanne/libquassel/protocol/variant/QVariantList.kt

...ocol/serializers/handshake/ClientInitAckSerializerTest.kt

...rotocol/serializers/handshake/ClientInitSerializerTest.kt