Data/Dialogs: Unify layout of prev/next buttons for page switching

Implements first part of ticket #2513.
TraceHistoryRenderer: Simplified sgn() function

This makes it possible to declare the function as constexpr
TraceHistoryRenderer: Make sgn() function constexpr
TraceHistoryRenderer: Moved sgn() function to fixed.hpp
fixed: Use std::lround() instead of lround()

std::lround() is constexpr, but lround() may not be.
(depending on the amount of compiler magic)
fixed: Make uround/iround() functions constexpr