[FL-1498] Keyboard save focus (#730)

* text_input: set focus on Save button if text is set
* text_input: add clear API, format documentation
* all: rework text input clean with new API

applications/nfc/scenes/nfc_scene_save_name.c

@@ -60,6 +60,5 @@ void nfc_scene_save_name_on_exit(void* context) {
     Nfc* nfc = (Nfc*)context;
 
     // Clear view
-    text_input_set_header_text(nfc->text_input, NULL);
-    text_input_set_result_callback(nfc->text_input, NULL, NULL, NULL, 0, false);
+    text_input_clean(nfc->text_input);
 }