kstyle: Fix small spinboxes
kstyle: Fix spinboxes affected by stylesheets
When an app uses a Qt stylesheet, it messes up the size hint. We switch to the old spinbox style in that situation to prevent the spinbox from being unusable.
kstyle: Make spinbox edit area fill the space between the buttons
This helps to prevent text from being cut off when there's still a bit of room.
kstyle: Make spinbox button separators as tall as the font height