Rename Screens to Activities and restructure files (#44)

## 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 😞

lib/Epub/Epub/Page.cpp

@@ -3,7 +3,9 @@
 #include <HardwareSerial.h>
 #include <Serialization.h>
 
+namespace {
 constexpr uint8_t PAGE_FILE_VERSION = 3;
+}
 
 void PageLine::render(GfxRenderer& renderer, const int fontId) { block->render(renderer, fontId, xPos, yPos); }
 

lib/Epub/Epub/Section.cpp

@@ -9,7 +9,9 @@
 #include "Page.h"
 #include "parsers/ChapterHtmlSlimParser.h"
 
+namespace {
 constexpr uint8_t SECTION_FILE_VERSION = 5;
+}
 
 void Section::onPageComplete(std::unique_ptr<Page> page) {
   const auto filePath = cachePath + "/page_" + std::to_string(pageCount) + ".bin";