Spinbox: use more appropriate icons for decrease and increase buttons
This is an alternative to !1176 (closed).
Previously they were using the list-remove and list-add icons, which was semantically wrong. It happened to work with Breeze icons because they were plain minus and plus signs. ...Until recently, when list-remove was changed to be an X, exposing the bug.
Instead, let's ask for dedicated spinbox-decrease and spinbox-increase icons, which are being added to Breeze (see frameworks/breeze-icons!390 (merged)). If these don't exist in the active icon theme, it will fall back to the existing icons and nothing will change.
3rd-party icon themes might still exhibit the bug if their list-add or list-remove icons look weird, but that's a pre-existing issue, and with this, they'll at least have a path to fix it.
BUG: 491312 FIXED-IN: 6.1.4