feat: Implement automatic IPC registration for CustomButton widgets

- Add automatic registration/unregistration of CustomButton instances to
CustomButtonIPCService
- Enable CustomButton widgets to register themselves with unique
identifiers
- Support external control via IPC commands using 'qs -c noctalia-loner
ipc call cb <action> "[identifier]"' format

Supported actions: left, right, middle, up (separate wheel mode), down
(separate wheel mode),wheel (unified wheel mode), refresh (textCommand)

shell.qml

@@ -133,6 +133,11 @@ ShellRoot {
         screenDetector: screenDetector
       }
 
+      // CustomButtonIPCService handles IPC commands for custom buttons
+      CustomButtonIPCService {
+        id: customButtonIPCService
+      }
+
       // Container for plugins Main.qml instances (must be in graphics scene)
       Item {
         id: pluginContainer