Project restructuring: add tactility-headless (#55)

- Created `tactility-headless` to support ESP32 firmwares that don't require graphics
- `tactility` subproject now contains both PC and ESP32 code (to avoid having to split up `tactility` and `tactility-headless` into separate projects, which would result in a very complex dependency tree)
- `tactility` subproject is now defined as component for ESP32 and as regular module for PC
- Improvements for dispatcher
- Added `project-structure.puml` to docs
- `Gui` service now depends on `Loader` service instead of the reverse
- Added `statusbar_updater` service for updating Wi-Fi and SD card icons

tactility-headless/src/service_manifest.h

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "tactility_core.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef void* Service;
+
+typedef void (*ServiceOnStart)(Service service);
+typedef void (*ServiceOnStop)(Service service);
+
+typedef struct {
+    /**
+     * The identifier by which the app is launched by the system and other apps.
+     */
+    const char* id;
+
+    /**
+     * Non-blocking method to call when service is started.
+     */
+    const ServiceOnStart _Nullable on_start;
+
+    /**
+     * Non-blocking method to call when service is stopped.
+     */
+    const ServiceOnStop _Nullable on_stop;
+
+} ServiceManifest;
+
+#ifdef __cplusplus
+}
+#endif