window.c: Add touch handlers

This adds basic infrastructure for handling touch events in the toytoolkit.
diff --git a/clients/window.h b/clients/window.h
index 3c60081..a817734 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -232,6 +232,26 @@
 					uint32_t button,
 					enum wl_pointer_button_state state,
 					void *data);
+typedef void (*widget_touch_down_handler_t)(struct widget *widget,
+					    uint32_t serial,
+					    uint32_t time,
+					    int32_t id,
+					    float x,
+					    float y,
+					    void *data);
+typedef void (*widget_touch_up_handler_t)(struct widget *widget,
+					  uint32_t serial,
+					  uint32_t time,
+					  int32_t id,
+					  void *data);
+typedef void (*widget_touch_motion_handler_t)(struct widget *widget,
+					      uint32_t time,
+					      int32_t id,
+					      float x,
+					      float y,
+					      void *data);
+typedef void (*widget_touch_frame_handler_t)(struct widget *widget,void *data);
+typedef void (*widget_touch_cancel_handler_t)(struct widget *widget, void *data);
 typedef void (*widget_axis_handler_t)(struct widget *widget,
 				      struct input *input, uint32_t time,
 				      uint32_t axis,
@@ -457,9 +477,23 @@
 widget_set_button_handler(struct widget *widget,
 			  widget_button_handler_t handler);
 void
+widget_set_touch_down_handler(struct widget *widget,
+			      widget_touch_down_handler_t handler);
+void
+widget_set_touch_up_handler(struct widget *widget,
+			    widget_touch_up_handler_t handler);
+void
+widget_set_touch_motion_handler(struct widget *widget,
+				widget_touch_motion_handler_t handler);
+void
+widget_set_touch_frame_handler(struct widget *widget,
+			       widget_touch_frame_handler_t handler);
+void
+widget_set_touch_cancel_handler(struct widget *widget,
+				widget_touch_cancel_handler_t handler);
+void
 widget_set_axis_handler(struct widget *widget,
 			widget_axis_handler_t handler);
-
 void
 widget_schedule_redraw(struct widget *widget);