feat: add private shell control protocol

compositor/src/private_client.rs

@@ -15,6 +15,7 @@ pub(crate) struct ClientCapabilities(u8);
 
 impl ClientCapabilities {
     pub(crate) const EFFECTS: Self = Self(1 << 0);
+    pub(crate) const SHELL_CONTROL: Self = Self(1 << 1);
 
     pub(crate) fn contains(self, capability: Self) -> bool {
         self.0 & capability.0 == capability.0
@@ -69,9 +70,15 @@ mod tests {
         let mut capabilities = ClientCapabilities::default();
         assert!(capabilities.is_empty());
         assert!(!capabilities.contains(ClientCapabilities::EFFECTS));
+        assert!(!capabilities.contains(ClientCapabilities::SHELL_CONTROL));
 
         capabilities.insert(ClientCapabilities::EFFECTS);
         assert!(!capabilities.is_empty());
         assert!(capabilities.contains(ClientCapabilities::EFFECTS));
+        assert!(!capabilities.contains(ClientCapabilities::SHELL_CONTROL));
+
+        capabilities.insert(ClientCapabilities::SHELL_CONTROL);
+        assert!(capabilities.contains(ClientCapabilities::EFFECTS));
+        assert!(capabilities.contains(ClientCapabilities::SHELL_CONTROL));
     }
 }