Reuse runner pane

[BanzaiMan]

Start the runner pane with remain-on-exit option, so that when an error is thrown during the Evaluate phase, the error is clearly indicated in the runner pane. We also respawn the runner pane when further edits are made in the editor pane. This allows the new output (or a new error) to be displayed in the runner pane.

'hsandbox' is terminated when the editor pane is terminated.