Learn "gaps edge_gaps <on|off|toggle>".

When yes, the old behaviour of adding half the inner gap around each
view is used.

When no, don't add any gap when an edge of the view aligns with the
workspace. The result is inner gap only between views, not against the
workspace edge.

The algorithm is not perfect because it means the extra space is
distributed amongst edge-aligned views only, but it's simple, looks good
and it works.

include/config.h

@@ -57,6 +57,7 @@ struct sway_config {
 	bool auto_back_and_forth;
 	bool seamless_mouse;
 
+	bool edge_gaps;
 	int gaps_inner;
 	int gaps_outer;
 };