Improve/calculator

[Yopai]

Some refactoring and improvements on the calculator widget

• fix HTML5 compliance (put style and script in the head. and comment after the DOCTYPE declaration)
• fix "floating point evil" when computing values near zero (like cos(pi/2))
• fix reusing results expressed in scientific notation
• Use pi as an alias for pi.
• Remove a ghostly "x" between the metas
• Make the widget responsive
• Use a true input, which allows to type 4*3+2,
  then select 3+2 and type or click a parenthese
  to have an expression that shows 4*(3+2)
• Refactor into a javascript object
• When computing, show the evaluated expression above the result
• Fix non working buttons (atan, ...)

You know what it is : you make a little modification... then you spend the day on it :), but I think it should preferable to use an existing library

see https://github.com/topics/javascript-calculator

[beyluta]

I'd have no issues using a library for this. This widget came as a submission so it wasn't up to me but I probably would have used one too. I'm working on a "note" widget using a library too :)