Cleanup of A2DPSource

[pschatzmann]

Today I have merged the cleanup of the A2DPSource:

• I removed the functionality to set some fixed audio data: please use the more flexible functionality of the AudioTools
• The preferred approach to start has changed: use the setter method to define the callback or data source and the start() w/o any data source arguments.
• I extended the data source functionality, so that you can indicate an Arduino stream or a callback which provides a new stream when the end has been reached: This way you can e.g. just use Files as data source