multimedia/arcan: recommend addons a la 8a400121ead9

Improve discoverability without circular dependency. While one can use
`arcan console` or install an appl (pipeworld, safespaces) out of band
the binary package already has many dependencies for extra features.

multimedia/arcan/pkg-message

@@ -0,0 +1,13 @@
+[
+{ type: install
+  message: <<EOM
+Additional packages recommended for new users:
+- durden (window manager and desktop environment)
+- xarcan (X11 bridge, similar to Xwayland)
+- aloadimage
+- acfgfs
+- aclip
+- arcan-trayicon
+EOM
+}
+]