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.