Make QPlatformTheme a gadget

So that we can wrap it and expose that wrapper to QML.

Also, make the ThemeHints enum available with Q_ENUM.

Change-Id: I920e7cc29c45c81c3a381b14ddb077f259f66b6c
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qplatformtheme.h

@@ -76,7 +76,9 @@ class QFileInfo;
 
 class Q_GUI_EXPORT QPlatformTheme
 {
+    Q_GADGET
     Q_DECLARE_PRIVATE(QPlatformTheme)
+
 public:
     Q_DISABLE_COPY_MOVE(QPlatformTheme)
 
@@ -121,6 +123,7 @@ public:
         InteractiveResizeAcrossScreens,
         ShowDirectoriesFirst,
     };
+    Q_ENUM(ThemeHint)
 
     enum DialogType {
         FileDialog,