CRM-16373 - Localization - Rearrange load/save logic

CRM-16373 - Localization - Rearrange load/save logic

 * Declare missing setting `languageLimit`
 * Allow saving admin form -- even when using makeMultilingual, makeSinglelingual, addLanguage, and/or languageLimit
 * Move onChange logic for `lcMessages` from `ConfigSetting::add()` to `onChangeLcMessages()`
 * Move locale init from `ConfigSetting::retrieve()` to `applyLocale()`