feat(rdp): printer redirection, saved as PostScript

An optional 'Nexus Terminal' printer (IronRDP's PostScript driver
announce) appears on the remote desktop; each job is written to
'Nexus print <time>.ps' in Downloads, with a toast saying where. Jobs
are capped at 256 MiB and empty jobs leave no file. The shared drive and
the printer now share one RDPDR backend (rdp_devices); the printer
works on every platform, the drive on Linux and macOS.

Tested with a unit test driving create/write/close; xrdp has no printer
redirection, so not exercised against a server.

rust/core/src/sessions/mod.rs

@@ -65,6 +65,11 @@ pub enum Event {
         session: String,
         error: String,
     },
+    /// Something the user might like to know (e.g. where a print job was saved).
+    Info {
+        session: String,
+        message: String,
+    },
     /// Something the user should know about that didn't stop the session (e.g. a credential that
     /// couldn't be saved).
     Warning {