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/FileSelectionActivity.h

@@ -0,0 +1,35 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+class FileSelectionActivity final : public Activity {
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  std::string basepath = "/";
+  std::vector<std::string> files;
+  int selectorIndex = 0;
+  bool updateRequired = false;
+  const std::function<void(const std::string&)> onSelect;
+  const std::function<void()> onGoHome;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+  void loadFiles();
+
+ public:
+  explicit FileSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+                                 const std::function<void(const std::string&)>& onSelect,
+                                 const std::function<void()>& onGoHome)
+      : Activity(renderer, inputManager), onSelect(onSelect), onGoHome(onGoHome) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};