WIP:endianness swap button

This was requested by a developer who uses gnome-calculator often. They said the only reason they still have to get their programmable calculator out sometimes while coding is because the programming mode doesn't have an easy way to swap endianness.

Still need to change a couple small things but wanted feedback on the idea. Testing so far does work on an answer but not on a number entered directly into the entry field -- I need to first get that number, swap it, and then set it as the answer.