Fixed joining and leaving buffers

...main/java/de/kuschku/libquassel/client/BufferManager.java

.../java/de/kuschku/libquassel/localtypes/BacklogFilter.java

...bquassel/syncables/types/abstracts/ABufferViewConfig.java

...schku/libquassel/syncables/types/impl/BacklogManager.java

...hku/libquassel/syncables/types/impl/BufferViewConfig.java

...ku/libquassel/syncables/types/impl/BufferViewManager.java

...a/de/kuschku/libquassel/syncables/types/impl/Network.java

...quassel/syncables/types/interfaces/QBufferViewConfig.java

...uassel/syncables/types/interfaces/QBufferViewManager.java