Add PreselectFirstFileInDirectory platform theme hint

This allows Qt Quick Dialogs to accurately check whether it should
select the first file in a directory when a file dialog is opened.

While we're here, fix an incorrect version on the last theme hint
that was added.

Task-number: QTBUG-98562
Change-Id: I08cc8a0fbed28a42d1212016c6ee7fc5459578bf
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -169,7 +169,11 @@ QT_BEGIN_NAMESPACE
 
     \value ShowDirectoriesFirst (bool) Whether directories should be shown
            first (before files) in file dialogs.
-           This enum value was added in Qt 6.2.
+           This enum value was added in Qt 6.3.
+
+    \value PreselectFirstFileInDirectory (bool) Whether the first file in a directory
+           should be automatically selected when a file dialog opens.
+           This enum value was added in Qt 6.3.
 
     \sa themeHint(), QStyle::pixelMetric()
 */
@@ -580,6 +584,8 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
         return true;
     case ShowDirectoriesFirst:
         return true;
+    case PreselectFirstFileInDirectory:
+        return false;
     }
     return QVariant();
 }

src/gui/kernel/qplatformtheme.h

@@ -122,6 +122,7 @@ public:
         MouseQuickSelectionThreshold,
         InteractiveResizeAcrossScreens,
         ShowDirectoriesFirst,
+        PreselectFirstFileInDirectory,
     };
     Q_ENUM(ThemeHint)
 

src/plugins/platformthemes/gtk3/qgtk3theme.cpp

@@ -135,6 +135,8 @@ QVariant QGtk3Theme::themeHint(QPlatformTheme::ThemeHint hint) const
         return QVariant(gtkSetting("gtk-icon-theme-name"));
     case QPlatformTheme::SystemIconFallbackThemeName:
         return QVariant(gtkSetting("gtk-fallback-icon-theme"));
+    case QPlatformTheme::PreselectFirstFileInDirectory:
+        return true;
     default:
         return QGnomeTheme::themeHint(hint);
     }