Add portable-file-dialogs submodule to 3rdparty.

.gitmodules

@@ -6,3 +6,7 @@
 	path = include/lol/3rdparty/mingw-std-threads
 	url = ../../lolengine/ext-mingw-std-threads.git
 	branch = master
+[submodule "portable-file-dialogs"]
+	path = include/lol/3rdparty/portable-file-dialogs
+	url = ../../samhocevar/portable-file-dialogs.git
+	branch = master

include/lol/3rdparty/portable-file-dialogs

@@ -0,0 +1 @@
+Subproject commit 5c6143956e27310668adec831628e896eddd8329

include/lol/base/dialogs.h

@@ -0,0 +1,21 @@
+//
+//  Lol Engine
+//
+//  Copyright © 2010—2020 Sam Hocevar <sam@hocevar.net>
+//
+//  Lol Engine is free software. It comes without any warranty, to
+//  the extent permitted by applicable law. You can redistribute it
+//  and/or modify it under the terms of the Do What the Fuck You Want
+//  to Public License, Version 2, as published by the WTFPL Task Force.
+//  See http://www.wtfpl.net/ for more details.
+//
+
+#pragma once
+
+//
+// The Portable File Dialogs classes
+// —————————————————————————————————
+//
+
+#include "../3rdparty/portable-file-dialogs/portable-file-dialogs.h"
+