v20 - light theme, history editing and more

theme lite and theme dark let you switch modes, so molencalc works better in a bright room, while still being easy on the eyes when it gets dark

As of v20, it's now possible to use the arrow keys to move through the current text you're editing, as well as moving back and forth through things you've already entered. Pretty basic stuff on the face of it, but it was surprisingly fiddly to get right...