Import should unescape backslash in LiteralString

[adaussy]

With the current implementation importing the following model:

package pk1 {
    part part1 {
        attribute a1 = "A \\ B";
        attribute a2 = "A \"a\"";
    }
}

Sets the value of "a1" = "A \ B" whereas it should be "A \ B" and "a2" = "A "a"" instead of "A "a"".