Reduced spacing between scrollbar arrows and groove for one arrow; reduce scrollbar padding slightly

(This is an updated version of !143 (closed), except with a more moderate reduction in the spacing)

I like the new wider Breeze scrollbar slider style for 5.23. However, with one button arrows enabled for scrollbars, I find that they don't look right and there is a large gap between the top of the groove and the arrow. This makes the scrollbar look like it isn't at an extremity when it is in fact at an extremity.

This merge request reduces this gap between the arrow and the groove (when one button arrow type is enabled). It also very slightly reduces the padding around the slider/groove while keeping the slider the same thickness to make the scrollbars more compact.

Scrollbars with one button arrows before this merge request (out of date screenshot -- since this was taken a thinner scrolbar groove was merged):

Scrollbars with one button arrows after this merge request:

With this change at 100% scaling you still have 288 square pixels clickable area for the arrow. This compares to 255 square pixels in Windows 10 default, and 225 square pixels in Chromium.