Store both filter boundaries with bookmarks

[Rua]

Currently, bookmarks store only one filter parameter, and the filter is always made symmetric. But in cases where a nearby station is causing a lot of interference on one side, it would be useful if both boundaries of the filter were stored.

If backwards compatibility with existing bookmarks files are a concern, filter width could remain a single field, but if there are two numbers with a separator between then this is interpreted as a low and high filter boundary. The low filter boundary would normally be a negative number, so the hyphen might not be the best choice for a separator; you might end up with things like -3000--500 for an LSB station and that would be confusing. Slash -3000/-500 or colon -3000:-500 might be better.