xpra: fix some runtime issues

* Add missing modules (fixes warnings and errors).
* Step 1 to unbreak starting Xvfb by making xpra invoke it with
  valid log dir ($HOME/.xpra). Without this fix, it is invoked with
  ~/.xpra, which Xvfb doesn't know how to interpret and uses it
  literally (fail). Step 2 will be fixing an Xvfb permission issue:
  "xf86OpenConsole: Cannot open virtual console 1 (Permission denied)".
* Use XPRA_INSTALL_PREFIX to make it find its icons.

pkgs/tools/X11/xpra/default.nix

@@ -29,7 +29,7 @@ buildPythonApplication rec {
   ];
 
   propagatedBuildInputs = with pythonPackages; [
-    pillow pygtk pygobject rencode
+    pillow pygtk pygobject rencode pycrypto cryptography pycups lz4 dbus
   ];
 
   preBuild = ''
@@ -47,6 +47,8 @@ buildPythonApplication rec {
     wrapProgram $out/bin/xpra \
       --set XKB_BINDIR "${xkbcomp}/bin" \
       --set FONTCONFIG_FILE "${fontsConf}" \
+      --set XPRA_LOG_DIR "\$HOME/.xpra" \
+      --set XPRA_INSTALL_PREFIX "$out" \
       --prefix LD_LIBRARY_PATH : ${libfakeXinerama}/lib \
       --prefix PATH : ${getopt}/bin:${xorgserver.out}/bin:${xauth}/bin:${which}/bin:${utillinux}/bin
   '';