Support for Filters

[danieldamon]

In the other versions of JS8Call, the application can be configured to focus on a particular bandpath via start and stop frequency filters.

A couple uses for this are:

1. filter lower than 1000 Hz to eliminate heartbeat chatter.
2. Filter a narrow bandwith to enhance communication with a group.
3. Filter a narrow bandwith to focus on replies to your CQ - supposing that the responder will reply on your frequency.