Statusbar improvements (#36)

- Added sdcard service: it updates a statusbar icon and it can auto-unmount sdcards that were physically ejected by the user.
- Added `is_mounted` check for sdcard drivers
- Refactored assets: moved them from `tactility-esp/` to `data/`
- Made assets work with sim build
- Refactored wifi statusbar icons
- Refactored wifi_manage app access point icons
- Support not having an initial image icon when registering a new icon in statusbars. When a statusbar icon is added, it is now visible/invisible depending on whether an image was specified.
- Keep track of app memory on app start to find memory leaks (needs fine-tuning in the future)
- `tt_init()` assigns `config_instance` earlier, so it can be used during service init

boards/yellow_board/sdcard.c

@@ -66,8 +66,14 @@ static void sdcard_unmount(void* context) {
     free(data);
 }
 
+static bool sdcard_is_mounted(void* context) {
+    MountData* data = (MountData*)context;
+    return (data != NULL) && (sdmmc_get_status(data->card) == ESP_OK);
+}
+
 const SdCard twodotfour_sdcard = {
     .mount = &sdcard_mount,
     .unmount = &sdcard_unmount,
+    .is_mounted = &sdcard_is_mounted,
     .mount_behaviour = SDCARD_MOUNT_BEHAVIOUR_ANYTIME
 };