fix(main): do not hang on exit

[imsnif]

There are two fixes here:

1. As per @TheLostLambda's request, we now have a more predictable close order for the threads. Waiting for one to close before closing the other in main.
2. Before, in the render method when we had no tabs left, we would send a quit message to the app. This caused a deadlock when we got a render instruction from the pty (eg. when there was some data left in its buffer) while quitting. To fix this, I moved this logic to the close_tab method, and then in the render method if there's no active tab it becomes a no-op.