evdev: Add mouse support

We know support EV_REL events comming from evdev devices. This addition
is pretty straigthforward, it adds a x,y per GSource listening to a
evdev device, updates from EL_REL (relative) events and craft new
ClutterMotionEvents. As for buttons, BTN_LEFT..BTN_TASK are translated
to ClutterButtonEvents with 1..8 as button number.

tests/interactive/test-events.c

@@ -187,8 +187,12 @@ input_cb (ClutterActor *actor,
               keybuf);
       break;
     case CLUTTER_MOTION:
-      g_print ("[%s] MOTION",
-               clutter_actor_get_name (source_actor));
+      {
+        ClutterMotionEvent *motion = (ClutterMotionEvent *) event;
+
+        g_print ("[%s] MOTION (%.02f,%.02f)",
+                 clutter_actor_get_name (source_actor), motion->x, motion->y);
+      }
       break;
     case CLUTTER_ENTER:
       g_print ("[%s] ENTER (from:%s)",