Copter: Added GCS feedback for save_trim()

Trim save gives no feedback, so I've added a gcs_send_text() call to
print "Trim saved" so the user knows to re-center trims again

ArduCopter/control_modes.pde

@@ -378,6 +378,7 @@ static void save_trim()
     float pitch_trim = ToRad((float)g.rc_2.control_in/100.0f);
     ahrs.add_trim(roll_trim, pitch_trim);
     Log_Write_Event(DATA_SAVE_TRIM);
+    gcs_send_text_P(SEVERITY_HIGH, PSTR("Trim saved"));
 }
 
 // auto_trim - slightly adjusts the ahrs.roll_trim and ahrs.pitch_trim towards the current stick positions