data-device: Don't implement data source through data offer object

The wl_data_source object used to specify the implementation for data
offers created for it.  This means you need a data offer to retrieve the
data from the source, which makes it awkward to use in-process in a
compositor.  Now we instead have three virtual functions that can be
connected to a protocol object or in-process data-sources such as an
X server proxy or clipboard.