river: implement xdg-activation-v1

- add a new "urgent" border color
- add a new event to river-status-unstable-v1

Co-authored-by: Isaac Freund <ifreund@ifreund.xyz>

completions/bash/riverctl

@@ -39,6 +39,7 @@ function __riverctl_completion ()
 			background-color \
 			border-color-focused \
 			border-color-unfocused \
+			border-color-urgent \
 			border-width \
 			focus-follows-cursor \
 			set-repeat \