Display brightness support (#26)

* cleanup

* brightness control and app

* cleanup

* persistant storage of display settings

* fix for missing include

* header cleanup

* fix pc build

* add docs

* move display app to tactility project

boards/yellow_board/yellow_board.c

@@ -1,4 +1,5 @@
 #include "yellow_board.h"
+#include "display_i.h"
 
 bool twodotfour_lvgl_init();
 bool twodotfour_bootstrap();
@@ -7,6 +8,9 @@ extern const SdCard twodotfour_sdcard;
 
 const HardwareConfig yellow_board_24inch_cap = {
     .bootstrap = &twodotfour_bootstrap,
+    .display = {
+        .set_backlight_duty = &twodotfour_backlight_set
+    },
     .init_lvgl = &twodotfour_lvgl_init,
     .sdcard = &twodotfour_sdcard
 };