[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

applications/lfrfid/scene/lfrfid-app-scene-select-key.h

@@ -0,0 +1,9 @@
+#pragma once
+#include "../lfrfid-app.h"
+
+class LfRfidAppSceneSelectKey : public GenericScene<LfRfidApp> {
+public:
+    void on_enter(LfRfidApp* app, bool need_restore) final;
+    bool on_event(LfRfidApp* app, LfRfidApp::Event* event) final;
+    void on_exit(LfRfidApp* app) final;
+};