Add XDG theme file for example icons

Allows easy use from the examples.

Change-Id: I731ea5641e247db2937aa1b12d1e91dd7dcb2848
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit e27841c2fb6b953ec9fda95bfc34d873801e51d8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/assets/icons/CMakeLists.txt

@@ -8,6 +8,7 @@ qt_internal_add_module(ExampleIconsPrivate
 )
 
 set(icons_resource_files
+    index.theme
     16x16/document-new.png
     16x16/document-open.png
     16x16/document-print.png
@@ -165,7 +166,7 @@ set(icons_resource_files
 
 qt_internal_add_resource(ExampleIconsPrivate "example_icons"
     PREFIX
-        "/qt-project.org/examples/icons/"
+        "/qt-project.org/icons/example_icons"
     FILES
         ${icons_resource_files}
 )

src/assets/icons/README

@@ -17,7 +17,13 @@ Setting up a project for using Example icon library
     )
     ...
 
-2. Create image resource in your application code:
+2. Load the theme
     ...
-    bool success = img->load(":/qt-project.org/examples/icons/32x32/document-new.png");
+    QIcon::setThemeSearchPaths(QIcon::themeSearchPaths() << u":/qt-project.org/icons"_s);
+    QIcon::setFallbackThemeName(u"example_icons"_s);
+    ...
+
+3. Use the icons
+    ...
+    const QIcon openIcon = QIcon::fromTheme("document-open");
     ...

src/assets/icons/index.theme

@@ -0,0 +1,46 @@
+[Icon Theme]
+Name=example_icons
+
+Directories=16x16,16x16@2,32x32,32x32@2,128x128,128x128@2,256x256,256x256@2,scalable
+
+[16x16]
+Size=16
+Type=Fixed
+
+[16x16@2]
+Size=16
+Scale=2
+Type=Fixed
+
+[32x32]
+Size=32
+Type=Fixed
+
+[32x32@2]
+Size=32
+Scale=2
+Type=Fixed
+
+[128x128]
+Size=128
+Type=Fixed
+
+[128x128@2]
+Size=128
+Scale=2
+Type=Fixed
+
+[256x256]
+Size=256
+Type=Fixed
+
+[256x256@2]
+Size=256
+Scale=2
+Type=Fixed
+
+[scalable]
+Size=512
+Type=Scalable
+MinSize=16
+MaxSize=512