Small request: String::insert

I suggest to add the following method to String. It would just call replaceSection() but that would help readability imho

/** Inserts another string right before the character indicated by the given index.

 Note that this is a const method, and won't alter the string itself.

 @param index               the index at which the string should be inserted.
 @param stringToInsert      the new string to insert at the given index.
String insert (int index, StringRef stringToInsert) const
    replaceSection (index, 0, stringToInsert);