#416 Improve usability of the save/load binary features

[Derek Fountain]

A patch to hopefully improve the usability of the save/load binary features.

Improvements:

• save filename, start and length in static context so those values can be offered as default next time
• don't immediately pop the filename selector if a filename is in the context, use that as default
• left align the filename in the dialog so it looks sensible if the dialog is resized wider
• put the widget focus in the start field when the dialog is popped up
• comment the code so I could understand it :)

[Sergio Baldoví]

Looks good to me. I'm keen to commit this.

[Sergio Baldoví]

Thanks. Committed in [4101f3].