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
Some handlers may want to call into other VMs (or even the one asking),
but vm.run() functions are coroutines, so needs to be called from
another coroutine. Allow for that.
Also fix typo in documentation.
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.
