Variable last_workspace moved to workspace object in global namespace

The variable last_workspace, that contains the last used workspace
is moved to the global workspace properties.

Now the screen is not needed to know the workspace_count.

The variable name is changed to workspace.last_used because a similar
variable name is also in the WApplication struct.

src/WindowMaker.h

@@ -484,6 +484,7 @@ extern struct wmaker_global_variables {
 	/* Workspace related */
 	struct {
 		int count;		/* number of workspaces */
+		int last_used;		/* last used workspace number */
 
 		WMFont *font_for_name;  /* used during workspace switch */
 	} workspace;