contentformats: Change the format of gdk_content_formats_to_string()

contentformats: Change the format of gdk_content_formats_to_string()

Now, we just print a whitespace-separated list of GTypes and mime types.

This makes this neat for 2 things:

1. Parsing it (see next commit)
2. Using it in GtkBuilder (see commits after that)

In particular, the common case of supporting a single GType (or mime
type) looks like just printing the GType (or mime type), which in
GtkBuilder looks like
<property name="formats">GdkTexture</property>