Also make clipboard_claim() a vfunc so backends can override it.
Because the whole operation a vfunc, backends have the option of adding
code before the actual claim is done and potentially even fail or do
something after the successful claim.
Instead of having just one function that has the gtype and mime type as
out arguments, have 3 functions: 1 that finds any match, 1 that finds a
GType match and one for a mime type match.
This makes the API way more convenient to use.
This requires implementing a "pipe" so we can have 2 streams running:
contentprovider => serializer => outputstream
inputstream => deserializer => reader
And the pipe shoves the data from the outputstream into the inputstream.
GdkContentProvider is the object that represents local data in the
clipboard.
This patch only introduces the object and adds the clipboard properties,
it does not yet provide a way for the actual implementations to access
it.
The only access that is implemented is the local shortcut GValue access.
This allows us not just to pass any mime type to the read function, but
it also makes it possible to pass multiple mime types and the clipboard
can then try them in order until it finds a supported one.
This is so far not implemented though.
