feat(wm): add border colour for monocle

This commit adds a new WindowKind to allow the user to set a unique
colour to identify when a window container in monocle mode is active.

resolve #404

komorebi-core/src/lib.rs

@@ -168,6 +168,7 @@ impl FromStr for SocketMessage {
 pub enum WindowKind {
     Single,
     Stack,
+    Monocle,
 }
 
 #[derive(