Add Qt icon library for examples

Icon library for file and text operations.
This is private library to be used with Qt examples.
Instructions on usage found in README file.

Task-number: QTBUG-110428
Change-Id: I762503c74aecc3a8efbf25877628cb8e85efe414
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 9c0e0e39f0a8a32feacd4bed4b006cadbeadb2b4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/CMakeLists.txt

@@ -71,6 +71,7 @@ add_subdirectory(tools)
 
 if(QT_FEATURE_gui)
     add_subdirectory(gui)
+    add_subdirectory(assets)
 
     if(QT_FEATURE_opengl)
         add_subdirectory(opengl)

src/assets/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(icons)