EPUB Reader:
- Short press BACK: go to file browser at the book's folder
- Long press BACK (1s+): go directly to home menu
File Browser:
- Short press BACK: go up one directory (or home if at root)
- Long press BACK (1s+): go directly to root folder
## Summary
* Give activities name and log when entering and exiting them
* Clearer logs when attempting to debug, knowing where users are coming
from/going to helps
## Summary
* This PR drastically reshapes the structure of the codebase, moving
from the concept of "Screens" to "Activities", restructing the files and
setting up the concept of subactivities.
* This should help with keep the main file clean and containing all
functional logic in the relevant activity.
* CrossPointState is now also a global singleton which should help with
accessing it from within activities.
## Additional Context
* This is probably going to be a bit disruptive for people with open
PRs, sorry 😞
