Feature Request: Option to add padding / border for image exports

[kmgallahan]

This is a feature quest to add the ability to apply padding when exporting images.

• [ ] The option to add percentage padding to exported images would be most useful
• [ ] The option to add pixel-based padding may be useful in some cases
• [ ] This could be configured in the Export settings to apply to all exports
• [ ] This could also optionally be applied in the Export flyout to apply to individual exports
• [ ] A preview of some sort in the Export settings to show the padding & character bounding boxes would be a useful stretch feature