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

boards/m5stack_core2/source/m5stack_core2.c

@@ -9,6 +9,6 @@ const HardwareConfig m5stack_core2 = {
     .display = {
         .set_backlight_duty = NULL
     },
-    .init_lvgl = &core2_lvgl_init,
+    .init_graphics = &core2_lvgl_init,
     .sdcard = &core2_sdcard
 };