Extract function to grab buttons when managing a window.

We refactor the button grabbing into a function to allow the next patch
both to
  - conditionally grab different sets of buttons
  - grab the buttons again when reloading the config.

relates to #2049

include/xcb.h

@@ -160,3 +160,9 @@ void xcb_add_property_atom(xcb_connection_t *conn, xcb_window_t window, xcb_atom
  *
  */
 void xcb_remove_property_atom(xcb_connection_t *conn, xcb_window_t window, xcb_atom_t property, xcb_atom_t atom);
+
+/**
+ * Grab the specified buttons on a window when managing it.
+ *
+ */
+void xcb_grab_buttons(xcb_connection_t *conn, xcb_window_t window, uint8_t* buttons);