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/settings/SettingsActivity.h

@@ -0,0 +1,42 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+#include "../Activity.h"
+
+class CrossPointSettings;
+
+// Structure to hold setting information
+struct SettingInfo {
+  const char* name;                        // Display name of the setting
+  uint8_t CrossPointSettings::* valuePtr;  // Pointer to member in CrossPointSettings
+};
+
+class SettingsActivity final : public Activity {
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  bool updateRequired = false;
+  int selectedSettingIndex = 0;  // Currently selected setting
+  const std::function<void()> onGoHome;
+
+  // Static settings list
+  static constexpr int settingsCount = 2;  // Number of settings
+  static const SettingInfo settingsList[settingsCount];
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+  void toggleCurrentSetting();
+
+ public:
+  explicit SettingsActivity(GfxRenderer& renderer, InputManager& inputManager, const std::function<void()>& onGoHome)
+      : Activity(renderer, inputManager), onGoHome(onGoHome) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};