When we eventually implement `workspace <ws> gaps inner|outer <px>`,
we'll need to store the gaps settings for workspaces before they're
created. Rather than create a workspace_gaps struct, the approach I'm
taking is to rename workspace_outputs to workspace_configs and then add
gaps settings to that.
I've added a lookup function workspace_find_config. Note that we have a
similar thing for outputs (output_config struct and output_find_config).
Lastly, when freeing config it would create a memory leak by freeing the
list items but not the workspace or output names inside them. This has
been rectified using a free_workspace_config function.
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