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
https://github.com/ChandhokTannay/crosspoint-reader/commit/1d36a86ef1f016e796ef993c14fb1f9ea2f0101d

lib/Epub/Epub.cpp

@@ -139,7 +139,7 @@ bool Epub::parseTocNcxFile() const {
 }
 
 // load in the meta data for the epub file
-bool Epub::load() {
+bool Epub::load(const bool buildIfMissing) {
   Serial.printf("[%lu] [EBP] Loading ePub: %s\n", millis(), filepath.c_str());
 
   // Initialize spine/TOC cache
@@ -151,6 +151,11 @@ bool Epub::load() {
     return true;
   }
 
+  // If we didn't load from cache above and we aren't allowed to build, fail now
+  if (!buildIfMissing) {
+    return false;
+  }
+
   // Cache doesn't exist or is invalid, build it
   Serial.printf("[%lu] [EBP] Cache not found, building spine/TOC cache\n", millis());
   setupCacheDir();