gtk3, gtk4: Progressbar and scale blue ocean

This commit introduces new style for progress bars and scales.

* A new function `gtkoverlay` is added to replicate `KColorUtils::overlayColors`
  used in Breeze QStyle using `linear-gradient` color overlaying
* ...and `gtkoverlayalpha` for the same thing plus alpha channel
* Scale bars with steps got tickmarks back
* Fixed broken Gnome Disks partition change scale margins

CCBUG: 443919