[FL-1371] New LF-RFID app. Second encounter. (#547)

* File worker: file operations helper.
* Notification app: removed yield
* File worker: write operations, calls to system file widgets
* App ibutton: use file worker
* Lfrfid: generic key loading, add path helper and hex conversion to lib
* FileWorker: plain C verison
* FileWorker: add to lib.mk
* FileWorker: add to C sources, instead of CPP
* Lfrfid: save scene
* App lfrfid: add key scene, saved key menu
* App lfrfid: saved key info scene
* App lfrfid: delete key scene

lib/args/hex.h

@@ -0,0 +1,20 @@
+#pragma once
+#include "stdint.h"
+#include "stdbool.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Convert ASCII hex value to nibble 
+ * 
+ * @param c ASCII character
+ * @param nibble nibble pointer, output
+ * @return bool conversion status
+ */
+bool hex_char_to_hex_nibble(char c, uint8_t* nibble);
+
+#ifdef __cplusplus
+}
+#endif