Revert default FlickDeceleration to 1500

At the time of bb1f616 the
Flickable.flickDeceleration property still applied to both touch
flicking and mouse wheel scrolling. In qtdeclarative
b1766d9d629f61d824146e69f1f3b319cbee3d11 we decoupled them. Switching
from the traditional 1500 logical pixels/sec² to 5000 was not enough to
satisfy those who complained about the mouse wheel "not being linear"
and at the same time made touch-flicking feel too sluggish. So let's
restore the traditional default deceleration value.

The flickDeceleration property is still adjustable, and the default
can still be overridden in any QPlatformTheme subclass.

[ChangeLog][QPA] The default value for the platform FlickDeceleration
hint is reverted to 1500 (rather than 5000 as in Qt 6.6). This sets
the default value for Flickable.flickDeceleration, which can be
overridden directly; and the default can also be overridden in any
QPlatformTheme subclass.

Task-number: QTBUG-35608
Task-number: QTBUG-35609
Task-number: QTBUG-52643
Task-number: QTBUG-97055
Fixes: QTBUG-121500
Change-Id: If52b61dfcd0c08a7c6e753f39dbe01f417e94bf4
Reviewed-by: Tor Arne Vestbø <[email protected]>
Reviewed-by: Richard Moe Gustavsen <[email protected]>
(cherry picked from commit ed66cf8)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

src/gui/kernel/qplatformtheme.cpp

@@ -638,7 +638,7 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
     case FlickMaximumVelocity:
         return QVariant(2500);
     case FlickDeceleration:
-        return QVariant(5000);
+        return QVariant(1500);
     case MenuBarFocusOnAltPressRelease:
         return false;
     case MouseCursorTheme: