Fixing bnc#869653: Adding using SLP: Filter is case-sensitive

Since the old code escaped most of the regexp characters I decided to go to s simple string compare using pure ruby. So I removed the escaping of the special characters in the filter_string. Instead of Builtins.regexpmatch I went for String::index which does not need character escaping.
`Convert.to_string` on a `UI.QueryWidget` can be omitted.