When pasting a value into calculator, remove major currency symbols (dollar, pound, euro, and yen). I'm not aware of anywhere in the source view that those symbols would be used (the currency conversion is outside the source view).
This issue was brought up in #224 (closed), but I believe that it is worth consideration. Even if we don't include all symbols, doesn't mean we shouldn't include the ones that are used commonly.
Pasting numbers with a currency symbol is a common use case for the calculator app. Both the Windows and Mac OS core calculator apps automatically remove the "$" symbol.
I don't have a background with vala, so hope my code is good.