Implement bar gaps

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.

sway/commands/bar.c

@@ -14,6 +14,7 @@ static struct cmd_handler bar_handlers[] = {
 	{ "colors", bar_cmd_colors },
 	{ "context_button", bar_cmd_context_button },
 	{ "font", bar_cmd_font },
+	{ "gaps", bar_cmd_gaps },
 	{ "height", bar_cmd_height },
 	{ "hidden_state", bar_cmd_hidden_state },
 	{ "icon_theme", bar_cmd_icon_theme },