InlineMediaDownloader: Fix callbacks

Found "thanks" to our great unit tests. There go like 16 hours of my
life.

Conflicts:
	src/InlineMediaDownloader.vala
InlineMediaDownloader: Port to write_pixbuf_async
InlineMediaDownloader: Handle deleted thumbnails

If the thumbnail is not available, we have to unfortunately load the
complete media from disk, create the thumbnail, write that and then use
the it.
InlineMediaDownloader: Fix indentation
InlineMediaDownloader: Catch remaining GErrors
InlineMediaDownloader: Add note if media is not downloadable
Utils: Add write_pixbuf_async function

Since write_to_stream_async is broken, we need to work around that.
Make more classes public

This will be needed anyways to they appear in our generated vapi, which
is needed for testing
InlineMediaDownloader: Use our extern gdk pixbuf functions

We are not depending on the newer valac YET, so async constructors can't
be used.
vapi/gdk-pixbuf: Add variadic argument to save_to_stream