[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.c

@@ -0,0 +1,16 @@
+#include "hex.h"
+
+bool hex_char_to_hex_nibble(char c, uint8_t* nibble) {
+    if((c >= '0' && c <= '9') || (c >= 'A' && c <= 'F') || (c >= 'a' && c <= 'f')) {
+        if((c >= '0' && c <= '9')) {
+            *nibble = c - '0';
+        } else if((c >= 'A' && c <= 'F')) {
+            *nibble = c - 'A' + 10;
+        } else {
+            *nibble = c - 'a' + 10;
+        }
+        return true;
+    } else {
+        return false;
+    }
+}