refactor code and two minor fixes

Refactor:
- Use a tokio runtime inside App instead of having an async main; and
  spawn tasks with the runtime instead of having another thread using
  two message channels.
- Move mounting and unmounting logic to Device instead of having a
  struct for each state.
- Client::get_block_devices is now in charge of the filtering of hidden
  devices and devices of a kind udiskstui doesn't care about. Devices
  implementing Filesystem that are cleartext devices for encrypted
  devices are also ignored. All handling of encrypted devices is done
  from the device implementing Encrypted.

Fixes:
- If an error occurs during exiting, cancel the exit and show the error
  inside the TUI instead of exiting and showing the errors in normal
  stdout
- Change device info after locking/unlocking.