QPA: add ButtonPressKeys ThemeHint

Platforms should allow more than just Qt::Key_Space and Qt::Key_Select
to press buttons. KDE Plasma developers want to be able to press buttons
with Return/Enter. GTK allows buttons to be pressed with Return/Enter,
so this should be considered the native behavior for GNOME as well.
Adding this ThemeHint allows customizing the keys that can press a
button via a QPlatformTheme subclass.

Change-Id: I73284e9b73590fbdd0b94a0cb4557e94b3722af9
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -175,6 +175,8 @@ QT_BEGIN_NAMESPACE
            should be automatically selected when a file dialog opens.
            This enum value was added in Qt 6.3.
 
+    \value ButtonPressKeys (QList<Qt::Key>) A list of keys that can be used to press buttons via keyboard input.
+
     \sa themeHint(), QStyle::pixelMetric()
 */
 
@@ -586,6 +588,8 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
         return true;
     case PreselectFirstFileInDirectory:
         return false;
+    case ButtonPressKeys:
+        return QVariant::fromValue(QList<Qt::Key>({ Qt::Key_Space, Qt::Key_Select }));
     }
     return QVariant();
 }

src/gui/kernel/qplatformtheme.h

@@ -123,6 +123,7 @@ public:
         InteractiveResizeAcrossScreens,
         ShowDirectoriesFirst,
         PreselectFirstFileInDirectory,
+        ButtonPressKeys
     };
     Q_ENUM(ThemeHint)