WIP: Adwaita: Fix and improve scale widget sizing

• Don't expand the scale widget itself that has a value when in fine-tune mode.
  • (Check the vertical scale in gtk4-widget-factory for example.)
• Don't jump the slider position when it is at the start or end and in fine-tune mode.
  • Here is the behavior in the current master: Peek_2020-04-18_16-24
• Center the unlabeled marked scale in the widget.
• Make the unlabeled marked scale the same width/height as the unmarked one.
• Remove the big padding around the scale with a value.

Before:

After:

For comparison, here is the scale widget sizing in GTK 3:

And here is the CSS snippet I used to visualize the sizing locally:

scale,
scale > marks,
scale > value {
  background-color: alpha(red, 0.2);
}