Add drag n drop support for audio files

[NeoCoderMatrix86]

Is your feature request related to a problem? Please describe.
Dropping audio files is not possible any more. Please enable dropping files and attaching them to the current cuesheet.

[NeoCoderMatrix86]

Dragging a file into the application should search for empty audiofiles and set the first audiofile beeing found. If not clear a mapping dialog should be displayed asking the user for making an assignment.