Remove all thumbnail prelighting code

Turns out that prelighting just works out of the box by setting
Gtk.CellRendererPixbuf.follow-state property to true.

This patch reverts these patches:

2531ed10bd8d326263b75671882d296d367f87c9 "machine: Fix prelight for non-running boxes".
fff165f47924eee0d1d375d3373f9fbef439c4fa "collection-view: Prelight for running machines"
d3f79d6fb5da1bb5966e8aaca6ea63e34f0c4a44 "collection-view: No prelight after mouse leaves view"
2b9d1314c0739a7a2f4737ef058bd7a5b4c0fc7b "collection-view: Implement prelight on mouse hover"
cf9e16f62ca729bc754626655c3040184aa5fa44 "machine: Make origin_pixbuf a public property"

Conflicts:
	src/collection-view.vala
	src/machine.vala
Bump to newest version for prelight to work