machine: Make origin_pixbuf a public property

We then use it from CollectionView in a following patch to implement
prelighting of item under the cursor.

https://bugzilla.gnome.org/show_bug.cgi?id=710279
build: Bump vala dep to 0.23.3.1 (i-e next release)

We already require a very recent version of unstable branch so requiring
a bit newer version to not have to work around a small issue in gtk+
binding is totally worth it IMO.

https://bugzilla.gnome.org/show_bug.cgi?id=710279
collection-view: Implement prelight on mouse hover

https://bugzilla.gnome.org/show_bug.cgi?id=710279