Implemented proper offline functionality handling

...java/de/kuschku/quasseldroid_ng/service/QuasselService.kt

...c/main/java/de/kuschku/quasseldroid_ng/ui/MainActivity.kt

lib/src/main/java/de/kuschku/libquassel/session/Backend.kt

...main/java/de/kuschku/libquassel/session/CoreConnection.kt

...ain/java/de/kuschku/libquassel/session/ICoreConnection.kt

lib/src/main/java/de/kuschku/libquassel/session/ISession.kt

...ain/java/de/kuschku/libquassel/session/ProtocolHandler.kt

lib/src/main/java/de/kuschku/libquassel/session/Session.kt

...main/java/de/kuschku/libquassel/session/SessionManager.kt

...src/test/java/de/kuschku/libquassel/ConnectionUnitTest.kt