Standalone app api v3 (#2772)

Added file io, and updated some ui elements.
Also added Digital Rain standalone app for an example.

firmware/standalone/digitalrain/ui/result.hpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (C) 2023 Kyle Reed
+ * Copyright (C) 2015 Jared Boone, ShareBrained Technology, Inc.
+ * Copyright (C) 2016 Furrtek
+ *
+ * This file is part of PortaPack.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include <utility>
+
+template <typename TValue, typename TError>
+struct Result {
+    enum class Type {
+        Success,
+        Error,
+    } type;
+
+    union {
+        TValue value_;
+        TError error_;
+    };
+
+    bool is_ok() const {
+        return type == Type::Success;
+    }
+
+    operator bool() const {
+        return is_ok();
+    }
+
+    bool is_error() const {
+        return type == Type::Error;
+    }
+
+    const TValue& value() const& {
+        return value_;
+    }
+
+    const TValue& operator*() const& {
+        return value_;
+    }
+
+    TValue&& operator*() && {
+        return std::move(value_);
+    }
+
+    const TError& error() const& {
+        return error_;
+    }
+
+    Result() = delete;
+
+    constexpr Result(TValue&& value)
+        : type{Type::Success},
+          value_{std::forward<TValue>(value)} {
+    }
+
+    constexpr Result(TError error)
+        : type{Type::Error},
+          error_{error} {
+    }
+
+    ~Result() {
+        if (is_ok())
+            value_.~TValue();
+        else
+            error_.~TError();
+    }
+};