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
Pick-to: 6.5
Change-Id: I762503c74aecc3a8efbf25877628cb8e85efe414
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

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)