Extract workspace size computation from render_workspace_button()

Also remove some unnecessary strtup()s and rename a few variables and functions.

include/bar/bar.h

@@ -32,6 +32,9 @@ struct workspace {
 	bool urgent;
 };
 
+/** Global bar state */
+extern struct bar swaybar;
+
 /**
  * Setup bar.
  */

include/bar/render.h

@@ -14,4 +14,9 @@ void render(struct output *output, struct config *config, struct status_line *li
  */
 void set_window_height(struct window *window, int height);
 
+/**
+ * Compute the size of a workspace name
+ */
+void workspace_button_size(struct window *window, const char *workspace_name, int *width, int *height);
+
 #endif /* _SWAYBAR_RENDER_H */