This is a GSList of GFile and we want it so we can operate with lists of
files and text/uri-list.
I chose GSList over GList because that's what the GtkFileChooser API
uses, too.
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 is not used by anything yet, but add it now, so people looking at
this new code can make sense of it.
Plus, the documentation mentions it, so better have the docs make sense.
It will be used once we add support for conversions to GDK and allow
doing cipboard/dnd by GValue.
Make sure the API reflects the idea that GdkContentFormats is a set
containing mime types. In particular, treat the object itself as a
plural - it's named content format`S' after all - and therefor use
the correct verb form.
Also make GdkContentFormats keep an array instead of a list, now that
it's immutable.
