In commit 4e41577b, we are using g_content_type_is_a() to determine how
to display the demo resources in the right pane of the gtk3-demo program.

Use g_content_type_get_mime_type(), so that we can obtain the mime
type of the demo resources on all platforms, as g_content_type_guess()
returns a platform-specific string, as
https://developer.gnome.org/gio/2.35/gio-GContentType.html states.

As .ui files and .css files are normally registered with a different mime
type string on Windows, check for those strings as well.

This will ensure the demo resources can be properly displayed on Windows
as well.