Allow lookup of Buffers by channel or username (for #quassel or irc:// handling)

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

...java/de/kuschku/libquassel/localtypes/buffers/Buffer.java

.../kuschku/libquassel/localtypes/buffers/ChannelBuffer.java

...de/kuschku/libquassel/localtypes/buffers/QueryBuffer.java

...e/kuschku/libquassel/localtypes/buffers/StatusBuffer.java

...kuschku/libquassel/syncables/types/impl/BufferSyncer.java

...ku/libquassel/syncables/types/interfaces/QIrcChannel.java

...schku/libquassel/syncables/types/interfaces/QIrcUser.java

...java/de/kuschku/quasseldroid_ng/ui/chat/ChatActivity.java

app/src/main/res/menu/chat.xml