[kcmkwin/kwinrules] Don't overflow spinbox for long strings

The spinbox should use a minimum width rather than a preferred width, so it can grow as needed. And once we do this, it's safe to make it a bit narrower by default, because its text contents will never overflow.

Short string now:

Long string now:

BUG: 438193 FIXED-IN: 5.22.1

@iasensio