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/file_reader.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2023 Kyle Reed
+ *
+ * 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 "file_reader.hpp"
+#include <string_view>
+#include <vector>
+
+/* Splits the string on the specified char and returns
+ * a vector of string_views. NB: the lifetime of the
+ * string to split must be maintained while the views
+ * are used or they will dangle. */
+std::vector<std::string_view> split_string(std::string_view str, char c) {
+    std::vector<std::string_view> cols;
+    size_t start = 0;
+
+    while (start < str.length()) {
+        auto it = str.find(c, start);
+
+        if (it == str.npos)
+            break;
+
+        cols.emplace_back(&str[start], it - start);
+        start = it + 1;
+    }
+
+    if (start <= str.length() && !str.empty())
+        cols.emplace_back(&str[start], str.length() - start);
+
+    return cols;
+}