Fix thumbnailer crash and memory leak

If the request timer is active when XfdesktopThumbnailer is finalized, the timer is never removed, which will cause a segfault. This can happen if the icon style is switched to file/launcher icons, and then is quickly switched to something else.

This also "modernizes" the GObject use in Xfdesktop thumbnailer, fixes a possible memory leak, and properly chains up to the parent class in dispose and finalize.