Share settings page helpers, add GUI cursor/scale API

- move create_trackbar_setting/update_trackbar_label/create_checkbox from
  settings_audio/display/graphics pages to core:settings_common module
- GUI scale trackbar shows the really applied scale, e.g. '3 (effective 2)'
- add gui.get_scale(), gui.get_max_scale(), gui.get_cursor_pos() (UI units);
  document that input.get_mouse_pos() returns window pixels
- InventoryView default color is defined once (InventoryView::DEFAULT_COLOR)
  instead of duplicated literals; 'color' attribute still overrides it

res/texts/ru_RU.txt

@@ -13,6 +13,7 @@ Description=Описание
 Converting world...=Выполняется конвертация мира...
 Unlimited=Неограниченно
 Auto=Авто
+Effective=действует
 Chat=Чат
 Console=Консоль
 Log=Лог