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 😞

src/activities/reader/ReaderActivity.cpp

@@ -0,0 +1,68 @@
+#include "ReaderActivity.h"
+
+#include <SD.h>
+
+#include "CrossPointState.h"
+#include "Epub.h"
+#include "EpubReaderActivity.h"
+#include "FileSelectionActivity.h"
+#include "activities/util/FullScreenMessageActivity.h"
+
+std::unique_ptr<Epub> ReaderActivity::loadEpub(const std::string& path) {
+  if (!SD.exists(path.c_str())) {
+    Serial.printf("[%lu] [   ] File does not exist: %s\n", millis(), path.c_str());
+    return nullptr;
+  }
+
+  auto epub = std::unique_ptr<Epub>(new Epub(path, "/.crosspoint"));
+  if (epub->load()) {
+    return epub;
+  }
+
+  Serial.printf("[%lu] [   ] Failed to load epub\n", millis());
+  return nullptr;
+}
+
+void ReaderActivity::onSelectEpubFile(const std::string& path) {
+  exitActivity();
+  enterNewActivity(new FullScreenMessageActivity(renderer, inputManager, "Loading..."));
+
+  auto epub = loadEpub(path);
+  if (epub) {
+    APP_STATE.openEpubPath = path;
+    APP_STATE.saveToFile();
+    onGoToEpubReader(std::move(epub));
+  } else {
+    exitActivity();
+    enterNewActivity(new FullScreenMessageActivity(renderer, inputManager, "Failed to load epub", REGULAR,
+                                                   EInkDisplay::HALF_REFRESH));
+    delay(2000);
+    onGoToFileSelection();
+  }
+}
+
+void ReaderActivity::onGoToFileSelection() {
+  exitActivity();
+  enterNewActivity(new FileSelectionActivity(
+      renderer, inputManager, [this](const std::string& path) { onSelectEpubFile(path); }, onGoBack));
+}
+
+void ReaderActivity::onGoToEpubReader(std::unique_ptr<Epub> epub) {
+  exitActivity();
+  enterNewActivity(new EpubReaderActivity(renderer, inputManager, std::move(epub), [this] { onGoToFileSelection(); }));
+}
+
+void ReaderActivity::onEnter() {
+  if (initialEpubPath.empty()) {
+    onGoToFileSelection();
+    return;
+  }
+
+  auto epub = loadEpub(initialEpubPath);
+  if (!epub) {
+    onGoBack();
+    return;
+  }
+
+  onGoToEpubReader(std::move(epub));
+}