Add enum class Qt::Appearance

It has been decided to add an appearance property in QStyleHints, which
will be propagated to classes that do not include QPlatformTheme.

Therefore an appearance enum class is added to the Qt namespace, thus
being available to all Qt classes.

Task-number: QTBUG-106383
Change-Id: Icff94b0d7adca954ce74241d6811401d41f053e6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/global/qnamespace.h

@@ -46,6 +46,12 @@ namespace Qt {
         transparent
     };
 
+    enum class Appearance {
+        Unknown = 0x0000,
+        Light   = 0x0001,
+        Dark    = 0x0002
+    };
+
     enum MouseButton {
         NoButton         = 0x00000000,
         LeftButton       = 0x00000001,
@@ -1745,6 +1751,7 @@ namespace Qt {
     Q_ENUM_NS(DayOfWeek)
     Q_ENUM_NS(CursorShape)
     Q_ENUM_NS(GlobalColor)
+    Q_ENUM_NS(Appearance)
     Q_ENUM_NS(AspectRatioMode)
     Q_ENUM_NS(TransformationMode)
     Q_FLAG_NS(ImageConversionFlags)

src/corelib/global/qnamespace.qdoc

@@ -788,6 +788,17 @@
     \sa QDockWidget::setAllowedAreas, QDockWidget::isAreaAllowed
 */
 
+/*!
+    \enum Qt::Appearance
+
+    Represents the appearance of an application's theme,
+    defined by QGuiApplication::palette().
+
+    \value Unknown The appearance is unknown.
+    \value Light The background colors are lighter than the text color, i.e. the theme is light.
+    \value Dark The background colors are darker than the text color, i.e. the theme is dark.
+*/
+
 /*!
     \enum Qt::ImageConversionFlag