Replace defunct exchange rates source and fix bitcoin exchange rate (libqalculate).

Ignore underscore in number (libqalculate).Add units for mean Gregorian and tropical years (libqalculate).Return gcd of numerators divided by lcm of denominators in gcd() with non-integer rational numbers, and vice versa for lcm() (libqalculate).Fix keypad button (up/down) for cycling through expression history.Add percent and permille output options in rational number form menu.Prefer capitalized function name (over name with underscore), when completing function name.Always add result to history (even if identical to previous) if expression contains a function generating a random number.Improve decision to (not) update calculate-as-you-type result when last character is an operator.Support for specifying a fixed denominator for display of fractions (e.g.Features include a large library of customizable functions, unit calculations and conversion, symbolic calculations (including integrals and equations), arbitrary precision, uncertainty propagation, interval arithmetic, plotting, and a user-friendly interface (GTK, Qt, and CLI). It is simple to use but provides power and versatility normally reserved for complicated math packages, as well as useful tools for everyday needs (such as currency conversion and percent calculation). Qalculate! is a multi-purpose cross-platform desktop calculator.