[kcmkwin/kwindesktop] Implement valueFromText in a spinbox

If textFromValue is set, valueFromText must be.

A lot of other implemenetations were using parseInt() which fails at
reading "3,000" due to the comma. Qt's fromLocaleString handles that but
needs any text stripping.

This implementation isn't perfect, but really we need to work on a good
upstream solution.

BUG: 437472


(cherry picked from commit 5484b846)