Make slaves send their module id to the master.

shared/slave_protocol.h

@@ -27,9 +27,18 @@
 
     typedef enum {
         SlaveProperty_Sync,
+        SlaveProperty_ModuleId,
         SlaveProperty_Features,
     } slave_property_t;
 
+    typedef enum {
+        ModuleId_LeftKeyboardHalf = 1,
+        ModuleId_KeyClusterLeft   = 2,
+        ModuleId_TrackballRight   = 3,
+        ModuleId_TrackpointRight  = 4,
+        ModuleId_TouchpadRight    = 5,
+    } module_id_t;
+
     typedef struct {
         uint8_t keyCount;
         bool hasPointer;