Adds the bar subcommand `gaps <amount>|<horizontal> <vertical>|<top>
<right> <bottom> <left>` to set gaps for swaybar. Due to restrictions on
margins for a layer_surface, only the sides that are anchored to an edge
of the screen can have gaps. Since there is support for per-side outer
gaps for workspaces, those should be able to be used instead for the
last side.
