First attempt at storing UI state for librifox in a dedicated object

librifox_ui_state.js

function UIState(args) {
  this.currentBook    = args.currentBook;
  this.currentChapter = args.currentChapter;
  this.bookCache      = args.bookCache; // did not hard-code the coupling with our global bookCache
  
  this.setCurrentBookById = function(id) {
    this.currentBook = this.bookCache[id];
    return this.currentBook; // should setter methods be returning values?
  }
  this.setCurrentChapterByIndex = function(index) {
    this.currentChapter = this.currentBook.chapters[index];
    return this.currentChapter; // should setter methods be returning values?
  }
}