citra-qt: Add a utility spinbox class called CSpinBox.

This class has a few advantages over the regular QSpinBox:
- QSpinBox stores its as signed 32 bit integers, which for instance is unsuitable for representing memory addresses. CSpinBox uses 64 bit integers instead.
- QSpinBox does not provide an easy way to handle number input from bases different than 10.
- QSpinBox is quite inflexible in general and almost any sort of customization requires reimplementing it anyway.