Clicking buttons that insert multi byte utf-8 characters makes the cursor skip positions

[santileortiz]

Pressing buttons for ×, ÷, −, √ and π while in the middle of a number is broken, it makes the cursor move ahead from it's actual position.

The reason is these are Unicode characters that get encoded into a multi byte string, then we increase the cursor position by the number of bytes not the number of Unicode characters.

Note that we can't just increase by 1 always, because trigonometric functions do contain multiple Unicode characters.