Add "transport" controls to the GUI

[nellump]

It would be nice to have both keyboard shortcuts and GUI buttons that enabled the user to step through their collection of bookmarks. For example:

• buttons that merely move to the next or previous bookmark
• buttons that move to the next or previous bookmark, and also send its values to the remote gqrx process

These buttons could be arranged intelligently so that it is obvious that some of them act on the previous or next bookmark, while existing buttons ("Add", "Delete", "Get") act on the currently selected bookmark.

I'd be happy to implement this.