Main changes:
● Fixed drawing of view-items with right-aligned decorations.
● Added an option for always centering normal tabs (in contrast to the document mode).
● Added an option (to Kvantum Manager) for styling vertical toolbars.
● Fixed a small problem in tab overlapping.
● Don't allow kinetic scrolling inside subwindows.
● Used version check for KWindowSystem because of method deprecations.
● Dropped support for Qt5 < 5.12.