Menu: conditional buttons, OpenScene returns the opened node

- MenuBuilder skips a button whose MenuButtonEntry.Visible predicate is
  false (e.g. "Continue" only when a last session exists); a throwing
  predicate is logged and the button stays visible.
- MenuBuilder.OpenScene returns the overlay it opened (null when the
  scene failed to build), so callers can configure it.
- Text for mods left out of a game's mod set: mods/not_in_game_reason.

locale/ui_en.properties

@@ -26,6 +26,7 @@ mods/enabled = Enabled
 mods/disabled = Disabled
 mods/disabled_reason = Disabled
 mods/dep_disabled_reason = Dependency "{0}" is disabled
+mods/not_in_game_reason = Not part of the game "{0}"
 mods/dependency_disabled = Dependency disabled
 mods/invalid_dep_format = Invalid dependency format: "{0}"
 mods/required_mod_not_loaded = Required mod "{0}" is not loaded

locale/ui_ru.properties

@@ -24,6 +24,7 @@ mods/enabled = Включён
 mods/disabled = Выключен
 mods/disabled_reason = Выключен
 mods/dep_disabled_reason = Зависимость "{0}" выключена
+mods/not_in_game_reason = Не входит в игру "{0}"
 mods/dependency_disabled = Зависимость отключена
 mods/invalid_dep_format = Неверный формат зависимости: "{0}"
 mods/required_mod_not_loaded = Требуемый мод "{0}" не загружен