Use enum wl_pointer_button_state instead of integer

Instead of using a uint32_t for state everywhere (except on the wire,
where that's still the call signature), use the new
wl_pointer_button_state enum, and explicit comparisons.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/clients/window.h b/clients/window.h
index a8537b3..9cef160 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -189,7 +189,8 @@
 				       float x, float y, void *data);
 typedef void (*widget_button_handler_t)(struct widget *widget,
 					struct input *input, uint32_t time,
-					uint32_t button, uint32_t state,
+					uint32_t button,
+					enum wl_pointer_button_state state,
 					void *data);
 
 struct window *