Variable workspace_count moved to the workspace object in the global namespace

The variable workspace_count, that contains the number of workspaces
is moved to the global workspace properties.

Now the screen is not needed to know the workspace_count.

src/WindowMaker.h

@@ -483,6 +483,8 @@ extern struct wmaker_global_variables {
 
 	/* Workspace related */
 	struct {
+		int count;		/* number of workspaces */
+
 		WMFont *font_for_name;  /* used during workspace switch */
 	} workspace;