Ruledit: Use QSpinBox for numeric requirement value
In requirement editing dialog, for requirement types that take numerical value, entry field is presented. The problem is that it's textual one (QLineEdit), and user can type in any characters, not just numbers. Make it QSpinBox instead.
- There's no such signal as ValueEntered for QSpinBox, and aftet changing that to working ValueChanged usability is very bad; we do a refresh after each digit is entered, unfocusing the field, i.e., one cannot type many digits at once
- Refactor code avoid loss of focus while editing the value