Fixes#621
* `ScreenInstruction::SetSelectable` etc.
were not updating correctly, if a NewTab was spawned, before
the plugin was finished setting the attributes.
Now the `tab_index` is used to send the instructions to
their respective tabs and plugins.
We add log4rs create for logging across Zellij. Additionally, we capture
`stderr` output from plugins and log it the same log file as other
Zellij logs.