Need way to cancel in-progress container creation

[Christopher Townsend]

Need a way to cancel in-progress container creation and clean up properly once canceled.

For the CLI, libertine-container-manager should catch the Ctrl-C and the SIGTERM and properly clean up.

For the GUI, if the container is in the installing state and a "Delete" action is performed, it should kill (SIGTERM) the libertine-container-manager QProcess and cleanup the spawned worker QThread.