Replace per-device touch slot with global slot

With multiple touch devices the slots could interfere

autotests/libinput/mock_libinput.cpp

@@ -635,7 +635,7 @@ double libinput_event_touch_get_y_transformed(struct libinput_event_touch *event
     return event->absolutePos.y() / deviceHeight * height;
 }
 
-int32_t libinput_event_touch_get_slot(struct libinput_event_touch *event)
+int32_t libinput_event_touch_get_seat_slot(struct libinput_event_touch *event)
 {
     return event->slot;
 }

libinput/events.cpp

@@ -263,7 +263,7 @@ qint32 TouchEvent::id() const
 {
     Q_ASSERT(type() != LIBINPUT_EVENT_TOUCH_CANCEL && type() != LIBINPUT_EVENT_TOUCH_FRAME);
 
-    const qint32 slot = libinput_event_touch_get_slot(m_touchEvent);
+    const qint32 slot = libinput_event_touch_get_seat_slot(m_touchEvent);
 
     return slot == -1 ? 0 : slot;
 }