Change default shortcut for Firefox in Linux

[coaxial]

Close #241

[fregante]

Thanks for the PR, but the suggestion was to drop the default shortcut altogether. However this requires further work to preserve it for existing installations. See:

• https://github.com/fregante/GhostText/pull/267

The next step is to remove this default, then iodate the code from the previous PR to:

1. get the saved shortcut from storage
2. set it back via chrome.commands API

The intent is:

• new users will not have command set because the manifest won't specify one
• old users will keep whatever was saved in memory before

[coaxial]

I see. When I opened the issue, I was only suggesting to change the default shortcut so it doesn't conflict with any existing one in Firefox.

I'll close this PR then as it doesn't do what you're planning.