3rdparty: add SortFilterProxyModel QML helper

Can be used directly in QML without having to derive from
QSortFilterProxyModel and manage source model settings from C++
code.

Change-Id: Icac1f6dadfc0d1f64b4459ae0cdbebdd30b79b3e

.gitmodules

@@ -3,6 +3,10 @@
 	url = https://github.com/BlueDragon747/qrencode-win32.git
 	ignore = dirty
 [submodule "sparkle/Sparkle"]
-        path = sparkle/Sparkle
-        url = https://github.com/sparkle-project/Sparkle.git
-        ignore = dirty
+	path = sparkle/Sparkle
+	url = https://github.com/sparkle-project/Sparkle.git
+	ignore = dirty
+[submodule "3rdparty/SortFilterProxyModel"]
+	path = 3rdparty/SortFilterProxyModel
+	url = https://github.com/atraczyk/SortFilterProxyModel.git
+	ignore = dirty

SortFilterProxyModel @ ccc46906

+Subproject commit ccc4690624709d558293aa08cb45e8edf30f73b4

INSTALL.md

@@ -94,10 +94,20 @@ Notes:
 
 ## Build only the client
 
-In order to use the Qt Client it is necessary to have the Qt version 5.14 or higher. If your system does not have it you can install it [from sources or download the binary installer](https://www.qt.io/download).
+In order to use the Qt Client it is necessary to have the Qt version 6.2 or higher. If your system does not have it you can install it [from sources or download the binary installer](https://www.qt.io/download).
 
 ## Build only this repository
 
+Clone with common required submodule (platform specific submodules will be cloned during the configure step)
+
+```bash
+git clone https://review.jami.net/jami-client-qt
+cd jami-client-qt
+git submodule update --recursive --init
+```
+
+Use CMake to build
+
 ```bash
 # In this repository
 mkdir build