Adds a validator to the search field

Author: CaptainDaVinci

src/main/java/org/jabref/gui/search/GlobalSearchBar.java

@@ -4,6 +4,7 @@
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.function.Function;
 import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;