improvements for audio uploader UX

When testing the audio uploader & its related permissions, there have been repeated remarks about the UX of the uploader and its related UI elements. Some basic improvement suggestions:

• for all uploader dialogues (except for file or import-from-url where a duration is calculated): to combine the upload dialog and the "enter duration dialog" into one UI would greatly improve the workflow, so users can directly enter a duration when uploading content.
• the "expert mode" checkbox is confusing users - think of a more intuitive solution and/or better UX writing to access the advanced audio source management (to be refined)
• the "enter m3u" dialog should give a hint that only entering a known filename from a file on the server is accepted as input (as long as this is the case)