inline some function of StateData and takes captures by rvalue (highlighter_benchmark 2.5% faster)

Inline some function of StateData and takes captures by rvalue to reduce copies (highlighter_benchmark 2.5% faster)

In addition:

- `Q_ASSERT(!isEmpty())` is removed because it duplicates the assertion in `m_contextStack.last()`.
- `QPair` is replaced by an internal structure (`StackValue`) to have more explicit names than first and second. This is also slightly more efficient.