Get rid of embedded browser (#255)

This change makes it possible to use Box, DropBox and Imgur with the default browser, instead of the embedded which causes many issues. Other plugins need to follow.

GreenshotPlugin/Core/OAuth/OAuth2AuthorizeMode.cs

@@ -0,0 +1,33 @@
+/*
+ * Greenshot - a free and open source screenshot tool
+ * Copyright (C) 2007-2020 Thomas Braun, Jens Klingen, Robin Krom
+ *
+ * For more information see: http://getgreenshot.org/
+ * The Greenshot project is hosted on GitHub https://github.com/greenshot/greenshot
+ *
+ * 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 1 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 <http://www.gnu.org/licenses/>.
+ */
+
+namespace GreenshotPlugin.Core.OAuth
+{
+    /// <summary>
+    /// Specify the authorize mode that is used to get the token from the cloud service.
+    /// </summary>
+    public enum OAuth2AuthorizeMode {
+        Unknown,		 // Will give an exception, caller needs to specify another value
+        LocalServer,	 // Will specify a redirect URL to http://localhost:port/authorize, while having a HttpListener
+        JsonReceiver,	 // Will start a local HttpListener and wait for a Json post
+        EmbeddedBrowser // Will open into an embedded _browser (OAuthLoginForm), and catch the redirect
+    }
+}