Re-write mono module editor code in C#

Make the build system automatically build the C# Api assemblies to be shipped with the editor.
Make the editor, editor player and debug export templates use Api assemblies built with debug symbols.
Always run MSBuild to build the editor tools and Api assemblies when building Godot.
Several bugs fixed related to assembly hot reloading and restoring state.
Fix StringExtensions internal calls not being registered correctly, resulting in MissingMethodException.

modules/mono/editor/GodotTools/GodotTools/Utils/OS.cs

@@ -0,0 +1,127 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Linq;
+using System.Runtime.CompilerServices;
+
+namespace GodotTools.Utils
+{
+    public static class OS
+    {
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        extern static string GetPlatformName();
+
+        const string HaikuName = "Haiku";
+        const string OSXName = "OSX";
+        const string ServerName = "Server";
+        const string UWPName = "UWP";
+        const string WindowsName = "Windows";
+        const string X11Name = "X11";
+
+        public static bool IsHaiku()
+        {
+            return HaikuName.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        public static bool IsOSX()
+        {
+            return OSXName.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        public static bool IsServer()
+        {
+            return ServerName.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        public static bool IsUWP()
+        {
+            return UWPName.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        public static bool IsWindows()
+        {
+            return WindowsName.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        public static bool IsX11()
+        {
+            return X11Name.Equals(GetPlatformName(), StringComparison.OrdinalIgnoreCase);
+        }
+
+        private static bool? _isUnixCache;
+        private static readonly string[] UnixPlatforms = {HaikuName, OSXName, ServerName, X11Name};
+
+        public static bool IsUnix()
+        {
+            if (_isUnixCache.HasValue)
+                return _isUnixCache.Value;
+
+            string osName = GetPlatformName();
+            _isUnixCache = UnixPlatforms.Any(p => p.Equals(osName, StringComparison.OrdinalIgnoreCase));
+            return _isUnixCache.Value;
+        }
+
+        public static char PathSep => IsWindows() ? ';' : ':';
+
+        public static string PathWhich(string name)
+        {
+            string[] windowsExts = IsWindows() ? Environment.GetEnvironmentVariable("PATHEXT")?.Split(PathSep) : null;
+            string[] pathDirs = Environment.GetEnvironmentVariable("PATH")?.Split(PathSep);
+
+            var searchDirs = new List<string>();
+
+            if (pathDirs != null)
+                searchDirs.AddRange(pathDirs);
+
+            searchDirs.Add(System.IO.Directory.GetCurrentDirectory()); // last in the list
+
+            foreach (var dir in searchDirs)
+            {
+                string path = Path.Combine(dir, name);
+
+                if (IsWindows() && windowsExts != null)
+                {
+                    foreach (var extension in windowsExts)
+                    {
+                        string pathWithExtension = path + extension;
+
+                        if (File.Exists(pathWithExtension))
+                            return pathWithExtension;
+                    }
+                }
+                else
+                {
+                    if (File.Exists(path))
+                        return path;
+                }
+            }
+
+            return null;
+        }
+
+        public static void RunProcess(string command, IEnumerable<string> arguments)
+        {
+            string CmdLineArgsToString(IEnumerable<string> args)
+            {
+                return string.Join(" ", args.Select(arg => arg.Contains(" ") ? $@"""{arg}""" : arg));
+            }
+
+            ProcessStartInfo startInfo = new ProcessStartInfo(command, CmdLineArgsToString(arguments))
+            {
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                UseShellExecute = false
+            };
+
+            using (Process process = Process.Start(startInfo))
+            {
+                if (process == null)
+                    throw new Exception("No process was started");
+
+                process.BeginOutputReadLine();
+                process.BeginErrorReadLine();
+            }
+        }
+    }
+}