Replace SingleApplication with KDSingleApplication

3rdparty/kdsingleapplication/README.md

@@ -0,0 +1,53 @@
+# KDSingleApplication
+
+`KDSingleApplication` is a helper class for single-instance policy applications
+written by [KDAB](https://www.kdab.com).
+
+## Usage
+
+Currently the documentation is woefully lacking, but see the examples or tests
+for inspiration. Basically it involves:
+
+1. Create a `Q(Core|Gui)Application` object.
+2. Create a `KDSingleApplication` object.
+3. Check if the current instance is *primary* (or "master") or
+   *secondary* (or "slave") by calling `isPrimaryInstance`:
+    * the *primary* instance needs to listen from messages coming from the
+      secondary instances, by connecting a slot to the `messageReceived` signal;
+    * the *secondary* instances can send messages to the primary instance
+      by calling `sendMessage`.
+
+## Licensing
+
+KDSingleApplication is (C) 2019-2023, Klarälvdalens Datakonsult AB, and is available
+under the terms of the [MIT license](LICENSES/MIT.txt).
+
+Contact KDAB at <info@kdab.com> if you need different licensing options.
+
+## Get Involved
+
+KDAB will happily accept external contributions.
+
+Please submit your contributions or issue reports from our GitHub space at
+<https://github.com/KDAB/KDSingleApplication>.
+
+## About KDAB
+
+KDSingleApplication is supported and maintained by Klarälvdalens Datakonsult AB (KDAB).
+
+The KDAB Group is the global No.1 software consultancy for Qt, C++ and
+OpenGL applications across desktop, embedded and mobile platforms.
+
+The KDAB Group provides consulting and mentoring for developing Qt applications
+from scratch and in porting from all popular and legacy frameworks to Qt.
+We continue to help develop parts of Qt and are one of the major contributors
+to the Qt Project. We can give advanced or standard trainings anywhere
+around the globe on Qt as well as C++, OpenGL, 3D and more.
+
+Please visit <https://www.kdab.com> to meet the people who write code like this.
+
+Stay up-to-date with KDAB product announcements:
+
+* [KDAB Newsletter](https://news.kdab.com)
+* [KDAB Blogs](https://www.kdab.com/category/blogs)
+* [KDAB on Twitter](https://twitter.com/KDABQt)