reduce the amount of information exposed by the proxy via dbus GetInfo

[totaam]

This was flagged as a potential issue during Bug 1102836 - AUDIT-0: xpra: suse-dbus-unauthorized-service org.xpra.Server a security review of xpra for packaging into suse.

FWIW: all the information exposed should normally already be accessible to all users on the system, but it is possible that we expose some internal attributes that should not be - difficult to tell, so let's err on the side of caution.

[totaam]

Done in r20186, 20187 backport for the v2.3.x branch.

Sample output from d-feet:

{'mode': 'proxy',
 'python': {'version': '3.6.6'},
 'session-type': 'proxy',
 'start_time': 65850,
 'type': 'Python',
 'uuid': 'ade8405288844b92aeddd5c073cd7097'}