Cleanup of c# api files and bindings generator

- We no longer generate RID and NodePath C# classes. Both will be maintained manually.
- We no longer generate C# declarations and runtime registration of internal calls for the following classes: RID, NodePath, String, GD, SignalAwaiter and Godot.Object (partial base).
- We no longer auto-generate the base members of Godot.Object. They will be maintained manually as a partial class.

This makes it easier to maintain these C# classes and their internal calls, as well as the bindings generator which no longer generates C# classes that don't derive from Godot Object, and it no longer generates the Godot.Object base members (which where unreadable in the bindings generator code).

- Added missing 'RID(Object from)' constructor to the RID C# class.
- Replaced MONO_GLUE_DISABLED constant macro with MONO_GLUE_ENABLED.
- Add sources in module/mono/glue even if glue is disabled, but surround glue files with ifdef MONO_GLUE_ENABLED.

modules/mono/glue/cs_files/Object.base.cs

@@ -0,0 +1,81 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Godot
+{
+    public partial class Object : IDisposable
+    {
+        private bool disposed = false;
+
+        private const string nativeName = "Object";
+
+        internal IntPtr ptr;
+        internal bool memoryOwn;
+
+        public Object() : this(false)
+        {
+            if (ptr == IntPtr.Zero)
+                ptr = godot_icall_Object_Ctor(this);
+        }
+
+        internal Object(bool memoryOwn)
+        {
+            this.memoryOwn = memoryOwn;
+        }
+
+        public IntPtr NativeInstance
+        {
+            get { return ptr; }
+        }
+
+        internal static IntPtr GetPtr(Object instance)
+        {
+            return instance == null ? IntPtr.Zero : instance.ptr;
+        }
+
+        ~Object()
+        {
+            Dispose(false);
+        }
+
+        public void Dispose()
+        {
+            Dispose(true);
+            GC.SuppressFinalize(this);
+        }
+
+        protected virtual void Dispose(bool disposing)
+        {
+            if (disposed)
+                return;
+
+            if (ptr != IntPtr.Zero)
+            {
+                if (memoryOwn)
+                {
+                    memoryOwn = false;
+                    godot_icall_Object_Dtor(this, ptr);
+                }
+
+                this.ptr = IntPtr.Zero;
+            }
+
+            disposed = true;
+        }
+
+        public SignalAwaiter ToSignal(Object source, string signal)
+        {
+            return new SignalAwaiter(source, signal, this);
+        }
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        internal extern static IntPtr godot_icall_Object_Ctor(Object obj);
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        internal extern static void godot_icall_Object_Dtor(object obj, IntPtr ptr);
+
+        // Used by the generated API
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        internal extern static IntPtr godot_icall_Object_ClassDB_get_method(string type, string method);
+    }
+}