Redesign home screen (#166)

## Summary

* Redesigned home screen with big option to continue reading and
slightly nicer options to navigate to core sections
* Attempt to use the cached EPUB details (title, author) if they exist,
otherwise fall back to file name
* Adjusted button hints on home screen, removed Back option and changed
left/right to up/down

## Additional Context

* Core of this work comes from @ChandhokTannay in
1d36a86ef1

lib/Epub/Epub.h

@@ -34,7 +34,7 @@ class Epub {
   }
   ~Epub() = default;
   std::string& getBasePath() { return contentBasePath; }
-  bool load();
+  bool load(bool buildIfMissing = true);
   bool clearCache() const;
   void setupCacheDir() const;
   const std::string& getCachePath() const;