Implemented the adjustable sleep timer feature as requested. Updated the settings storage to include refresh interval and default folder preferences. Modified the state file version to accommodate new fields for last browsed folder.
## Summary
* Swap to updated SDCardManager which uses SdFat
* Add exFAT support
* Swap to using FsFile everywhere
* Use newly exposed `SdMan` macro to get to static instance of
SDCardManager
* Move a bunch of FsHelpers up to SDCardManager
## 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 😞
