refactor font caching to just save the ID instead of mainting a cache with pattern→id-mapping

src/handlers.c

@@ -336,8 +336,7 @@ int handle_configure_request(void *prophs, xcb_connection_t *conn, xcb_configure
     DLOG("Configure request!\n");
     if (con_is_floating(con) && con_is_leaf(con)) {
         /* find the height for the decorations */
-        i3Font *font = load_font(conn, config.font);
-        int deco_height = font->height + 5;
+        int deco_height = config.font.height + 5;
         /* we actually need to apply the size/position changes to the *parent*
          * container */
         Rect bsr = con_border_style_rect(con);