Move remaining code to app module

There's still some stuff that could potentially go elsewhere but I think it's enough for now.

src/app/blurhash.h

@@ -0,0 +1,28 @@
+// SPDX-FileCopyrightText: 2018 Wolt Enterprises
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <stdint.h>
+
+/**
+ * @brief Returns the pixel array of the result image given the blurhash string.
+ *
+ * @param blurhash a string representing the blurhash to be decoded.
+ * @param width the width of the resulting image.
+ * @param height the height of the resulting image.
+ * @param punch the factor to improve the contrast, default = 1.
+ * @param nChannels the number of channels in the resulting image array, 3 = RGB, 4 = RGBA.
+ *
+ * @return A pointer to memory region where pixels are stored in (H, W, C) format.
+ */
+uint8_t *decode(const char *blurhash, int width, int height, int punch, int nChannels);
+
+/**
+ * @brief Checks if the Blurhash is valid or not.
+ *
+ * @param blurhash a string representing the blurhash.
+ *
+ * @return A bool (true if it is a valid blurhash, else false).
+ */
+bool isValidBlurhash(const char *blurhash);