Add two shortcuts:
- Ctrl+o to turn the device screen off while mirroring
- Ctrl+Shift+o to turn it back on
On power on (either via the POWER key or BACK while screen is off), both
the device screen and the mirror are turned on.
<https://github.com/Genymobile/scrcpy/issues/175>
After the recent refactorings, a "control event" is not necessarily an
"event" (it may be a "command"). Similarly, the unique "device event"
used to send the device clipboard content is more a "reponse" to the
request from the client than an "event".
Rename both to "message", and rename the message types to better
describe their intent.
