"Export" broken in Firefox

[ssorallen]

Describe the bug
Clicking the "Export" button in Firefox shows an alert but does not create a file.

To Reproduce
Steps to reproduce the behavior:

1. Open Tab Wrangler's preferences
2. Click on the "Options" tab
3. Scroll and click "Export"

The console shows an exception:

TypeError: e.props.dispatch(...) is undefined

...which I believe comes from

https://github.com/tabwrangler/tabwrangler/blob/master/app/js/OptionsTab.js#L153-L163

[ssorallen]

This was first reported in #104 (comment)

[ssorallen]

@ingorichter I discovered the issue: the export function is not returning a Promise like the import is. Also, the exporting is currently being done by the background script; that works in Chrome but not in Firefox. I am going to move the actual file creation code into the popup script.

[ssorallen]

Fixed in 3ed51b5. This will go into the next release, v6.5.0.

[ingorichter]

Thank you for fixing the issue @ssorallen 👏