Allow reversing spinbox drag and scroll directions and fix scroll directionality

Commit 1: Allow reversing spinbox drag and scroll directions

Sometimes you want dragging down or left or scrolling down to
increase the value of the spinbox. Examples include RTL support or a
place when dragging the spinbox expands or contracts a UI element and you
want the UI element to expand or contract in the direction of the drag.

Commit 2: Fix logic error in scroll directionality

Scrolling up was decreasing the value, when the reverse should be true.

These commits are a part of fixing https://bugs.kde.org/show_bug.cgi?id=429063.