Improved code, refactored it a bit, implemented full null safety

QuasselDroidNG/build.gradle

QuasselDroidNG/src/main/AndroidManifest.xml

...idNG/src/main/java/de/kuschku/libquassel/BusProvider.java

...elDroidNG/src/main/java/de/kuschku/libquassel/Client.java

...oidNG/src/main/java/de/kuschku/libquassel/ClientData.java

...G/src/main/java/de/kuschku/libquassel/CoreConnection.java

...src/main/java/de/kuschku/libquassel/IProtocolHandler.java

.../src/main/java/de/kuschku/libquassel/ProtocolHandler.java

...de/kuschku/libquassel/backlogmanagers/BacklogManager.java

...chku/libquassel/backlogmanagers/SimpleBacklogManager.java

...va/de/kuschku/libquassel/events/BacklogReceivedEvent.java

...java/de/kuschku/libquassel/events/BufferUpdatedEvent.java

...chku/libquassel/events/BufferViewManagerChangedEvent.java

...ava/de/kuschku/libquassel/events/BuffersChangedEvent.java

...a/de/kuschku/libquassel/events/ConnectionChangeEvent.java

...va/de/kuschku/libquassel/events/CoreSetupFailedEvent.java

...e/kuschku/libquassel/events/CoreSetupSuccessfulEvent.java

.../java/de/kuschku/libquassel/events/GeneralErrorEvent.java

...va/de/kuschku/libquassel/events/HandshakeFailedEvent.java

...n/java/de/kuschku/libquassel/events/LoginFailedEvent.java