[Development] WheelDeceleration for PlatformTheme and Flickable

[Wang Gary]

Hi,

I have two patchset related to QML's mouse wheel deceleration support,
which allow the developer fine-tune the mouse wheel deceleration
behavior when needed. One patch adds an enum for PlatformTheme-level
support which is for desktop-environment developers, and another patch
adds a property to Flickable which is for application developers.
There are the links to the two patches:

https://codereview.qt-project.org/c/qt/qtbase/+/557906
https://codereview.qt-project.org/c/qt/qtdeclarative/+/557907

These two patches have been here for around two months and no further
updates, thus kindly asking for review.

Also, there was a similar patch set for the same goal, but also had no
further updates for a year and then abandoned. I'll also put a link
here as a reference:

https://codereview.qt-project.org/c/qt/qtbase/+/436025

Regards,
Gary