Add "current-source" property

[Rafostar]

It would probably be useful to know (by showing it in "info" window) what current source is used, since there are already multiple possibilities (filesrc, giosrc, multiple http source elements, cddasrc, etc.).

Additionally, it could theoretically be used as a custom data feed (by e.g. setting media item URI to appsrc://, then getting source element and feeding data to it).