Workaround for parsing values in old PropertySheets

When you use a property sheet you can have a propertyrule with a value that starts with a number. For examples:

rule[doesntmatter]
{

name: 44something;
}

While technically, a string value starting with a number shouldn't need to be quoted, the parser fails under this condition. As a workaround, I've changed the quoting algorithm for output to quote string values unless they start with a letter.