Remove async bus callback from transcoder. This caused the UI to get unresponsive after a song was converted.

A bit more information:
Normal transcoding worked fine, the transcoder object was in the main thread. In the network remote, a new transcoder object is created which runs in the network remote thread. When a song was transcoded there, the UI became unresponsive.
Removing the async bus callback solved the problem, exactly the g_source_remove() method caused it. Since the bus callback returned FALSE (GST_BUS_DROP), the watch was already removed. (See http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstBus.html#gst-bus-add-watch)
Automatic merge of translations from Transifex (https://www.transifex.com/projects/p/clementine/resource/clementineplayer)
Cancel transcode when downloader disconnects.