glocalfileinfo: add dbus thumbnail generation requests

I've been sitting on this patch for years and honestly I don't even know if this is a sane thing for glib to do. I just figured I might as well finally open up an MR upstream and get feedback.

Basically this just lets glib send thumbnailing requests so that way dialog pickers can automatically generate thumbnails. e.g. if you get an image in someway without invoking the file manager (say wget url), then try to open up the file upload dialog in gtk4, there won't be a thumbnail for that image. This allows it to be generated in that case.