Add a public constructor to StringCursor

[benoitryder]

StringCursor fields being private is the only thing preventing a user to define its own EditableText, unless it directly wraps an existing one.

As an exemple, with this patch it is possible to wrap a RichText, implement few traits, and use it in a TextBox.

[xStrom]

Looks reasonable, just need a slight change to the changelog.

[xStrom]

Thanks!