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/boot_sleep/SleepActivity.cpp

@@ -0,0 +1,24 @@
+#include "SleepActivity.h"
+
+#include <GfxRenderer.h>
+
+#include "CrossPointSettings.h"
+#include "config.h"
+#include "images/CrossLarge.h"
+
+void SleepActivity::onEnter() {
+  const auto pageWidth = GfxRenderer::getScreenWidth();
+  const auto pageHeight = GfxRenderer::getScreenHeight();
+
+  renderer.clearScreen();
+  renderer.drawImage(CrossLarge, (pageWidth - 128) / 2, (pageHeight - 128) / 2, 128, 128);
+  renderer.drawCenteredText(UI_FONT_ID, pageHeight / 2 + 70, "CrossPoint", true, BOLD);
+  renderer.drawCenteredText(SMALL_FONT_ID, pageHeight / 2 + 95, "SLEEPING");
+
+  // Apply white screen if enabled in settings
+  if (!SETTINGS.whiteSleepScreen) {
+    renderer.invertScreen();
+  }
+
+  renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+}