filterString

Returns a new map containing only entries that match the predicate.

Example:

    var map = new HashMap<string, string> (GLib.str_hash, GLib.str_equal);
    map.put ("name", "Alice");
    map.put ("age", "30");
    map.put ("note", "");
    var filtered = Maps.filterString (map, (k, v) => {
        return v.length > 0;
    });
    // filtered contains only "name" and "age"

Parameters:

map	the source map.
fn	the predicate taking key and value.

Returns:

a new filtered HashMap.