Calculator: Add adding/subtracting/multiplying/dividing by a percentage

It's now possible to easily calculate 50% of 50. :^)

Userland/Applications/Calculator/Calculator.cpp

@@ -12,6 +12,12 @@
 
 Optional<Crypto::BigFraction> Calculator::operation_with_literal_argument(Operation operation, Crypto::BigFraction argument)
 {
+    // Support binary operations with percentages, for example "2+3%" == 2.06
+    if (m_binary_operation_in_progress != Operation::None && operation == Operation::Percent) {
+        argument = m_binary_operation_saved_left_side * Crypto::BigFraction { 1, 100 } * argument;
+        operation = Operation::None; // Don't apply the "%" operation twice
+    }
+
     // If a previous operation is still in progress, finish it
     // Makes hitting "1+2+3=" equivalent to hitting "1+2=+3="
     if (m_binary_operation_in_progress != Operation::None) {
@@ -20,7 +26,8 @@ Optional<Crypto::BigFraction> Calculator::operation_with_literal_argument(Operat
 
     switch (operation) {
     case Operation::None:
-        VERIFY_NOT_REACHED();
+        m_current_value = argument;
+        break;
 
     case Operation::Add:
     case Operation::Subtract: