Build icons dictionary from all provided icons, not only those allowed
as target. Specifically, source may not be allowed target (but the
window need its icon).
This way it will work independently from where qrexec-policy tool will
be called (in most cases - from a system service, as root).
This is also very similar architecture to what we'll need when moving to
GUI domain - there GUI part will also be separated from policy
evaluation logic.
QubesOS/qubes-issues#910
