It makes a lot of sense to call long-running operations in that event
handler, including calling back into the VM. Allow that by using
fire_event_async, not just fire_event.
Also, document the event.
Handle 'os' feature - if it's Windows, then set rpc-clipboard feature.
Handle 'gui-emulated' feature - request for specifically stubdomain GUI.
With 'gui' feature it is only possible to enable gui-agent based on, or
disable GUI completely.
Handle 'default-user' - verify it for weird characters and set
'default_user' property (if wasn't already set).
QubesOS/qubes-issues#3585
Make qubes.NotifyTools reuse logic of qubes.FeaturesRequest, then move
actual request processing to 'features-request' event handler. At the
same time implement handling 'qrexec' and 'gui' features request -
allowing to set template features when wasn't already there.
Behavior change: template is no longer allowed to change feature value
(regardless of being True or False). This means the user will always be
able to override what template have set.
