Make UiStyle::styleString() and friends static

styleString() actually just converts our internal format codes into a list
of FormatTypes, which are only used as keys for getting a particular format from
a particular style. Hence, styleString() itself can be made static. Amongst other things,
this has the advantage that StyledMessage's lazy styling can be encapsulated properly, and
there is no need to externally ensure that the message has been styled.