Move everything configuration related to plugin

If it's tied to the .so file rather than the .dll file it wouldn't make
any sense to use it directly from the Wine host.

src/plugin/configuration.h

@@ -0,0 +1,52 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// 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 3 of the License, 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.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include <filesystem>
+#include <optional>
+
+/**
+ * Starting from the starting file or directory, go up in the directory
+ * hierarchy until we find a file named `filename`.
+ *
+ * @param filename The name of the file we're looking for. This can also be a
+ *   directory name since directories are also files.
+ * @param starting_from The directory to start searching in. If this is a file,
+ *   then start searching in the directory the file is located in.
+ * @param predicate The predicate to use to check if the path matches a file.
+ *   Needed as an easy way to limit the search to directories only since C++17
+ *   does not have any built in coroutines or generators.
+ *
+ * @return The path to the *file* found, or `std::nullopt` if the file could not
+ *   be found.
+ */
+template <typename F = bool(const std::filesystem::path&)>
+std::optional<std::filesystem::path> find_dominating_file(
+    const std::string& filename,
+    std::filesystem::path starting_dir,
+    F predicate = std::filesystem::exists) {
+    while (starting_dir != "") {
+        const std::filesystem::path candidate = starting_dir / filename;
+        if (predicate(candidate)) {
+            return candidate;
+        }
+
+        starting_dir = starting_dir.parent_path();
+    }
+
+    return std::nullopt;
+}