When a view unmaps, we call workspace_consider_destroy. This function
assumed the workspace would always have an output, but this is not the
case when hotplugged down to zero. The function now handles this and
allows itself to be destroyed when there is no output.
This means that workspace_begin_destroy must remove the workspace from
the root->saved_workspaces list to avoid an eventual dangling pointer,
so it does that now.
Lastly, when an output is plugged in again and it has to create a new
initial workspace for it, we must emit the workspace::init IPC event
otherwise swaybar shows no workspaces at all. I guess when you start
sway, swaybar is started after the workspace has been created which is
why this hasn't been needed earlier.
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