For a session to actually persist, we need to request a persistence mode
which we already do. The portal then returns a restore-token (in the
form of an uuid) to us as part of the response to Start.
This token must then be passed into the *next* session during
SelectDevices to restore the previous session.
The token is officially a single-use token, so we need to overwrite it
every time. In practise the current XDP implementation may re-use the
token but we cannot rely on that.
Reading and writing the token is not async since we expect them to be
uuid-length.
Closes#74.
* Input capture and emulation can now be disabled and will prompt the user to enable again.
* Improved error handling to deliver more useful error messages
