dring interface: use queue of lambdas to wrap API calls

- changes the client-to-daemon interface to use a queue of lambdas
  instead of a enumerated task type parsed with a switch statement

- resulting code is: less duplicated, easier to read and modify

- better seperates the UI thread code

Change-Id: Id13defbab9ba36c3a2e51b3845de59ddd6667aaf
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>