feat: remember parent folder index in menu when ascending folders (#260)

## Summary

Adds feature to file selection activity for better user navigation when
ascending folders. The activity now remembers the index of the parent
folder instead of always resetting to the first element.

I don't have any means of testing this, so if someone could test it
that'd be great

Resolves #259

src/activities/reader/FileSelectionActivity.h

@@ -23,6 +23,7 @@ class FileSelectionActivity final : public Activity {
   [[noreturn]] void displayTaskLoop();
   void render() const;
   void loadFiles();
+  int findEntry(const std::string& name) const;
 
  public:
   explicit FileSelectionActivity(GfxRenderer& renderer, MappedInputManager& mappedInput,