Implement EWMH number of desktops property

_NET_NUMBER_OF_DESKTOPS:

> This property SHOULD be set and updated by the Window Manager to
> indicate the number of virtual desktops.

We interpret this property as the number of noninternal workspaces.

include/ewmh.h

@@ -18,6 +18,12 @@
  */
 void ewmh_update_current_desktop(void);
 
+/**
+ * Updates _NET_NUMBER_OF_DESKTOPS which we interpret as the number of
+ * noninternal workspaces.
+ */
+void ewmh_update_number_of_desktops(void);
+
 /**
  * Updates _NET_ACTIVE_WINDOW with the currently focused window.
  *