Properties defined with Props.Set() aren't escaped

In line of our earlier [discussion](https://github.com/emersion/go-ical/issues/6), escaping should happen without the user having to think about it. At the moment this isn't yet the case. The following two pieces of code have a different result:

```go
e.Props.Set(&ical.Prop{
Name: ical.PropDescription,
Value: `Hello world!\n-:-;`,
})
// produces: Hello world!\n-:-;
```
```go
e.Props.SetText(ical.PropDescription, `Hello world!\n-:-;`)
// produces: Hello world!\\n-:-\;
```