lookingGlass: bring back the inspector icon

lookingGlass: bring back the inspector icon

The change to StTextureCache for bug 644142 broke lg's inspector icon,
which was not specifying St.IconType.FULLCOLOR, but was relying on the
fact that SYMBOLIC (the default) would fall back to it. Fix the icon
by specifying FULLCOLOR explicitly.

(We should probably be using a symbolic icon here, but there is no
available icon with a select/pick/point-to/etc kind of meaning.)

https://bugzilla.gnome.org/show_bug.cgi?id=646451