Better handling of millisecond timestamps

The addon will now try to guess if it's decoding a unixtimestamp with or without milliseconds.
You can keep CTRL pressed while rightclicking to reverse this guess.

Please let me know if you find this helpful, or if you think it can be made smarter (it probably can :))

- The result of the timestamp decode is now shown directly in the context menu (only the non microsecond version)
- Time since timestamp is displayed
- If the selection isn't a number, the context menu now isn't shown
- Removed some unused code, smaller package!

Reverted back to only having one button, to avoid the dropdown
When decoding the timestamp, the timestamp will be decoded as if it was without milliseconds, and again as if it was with milliseconds, both results are displayed

-You can now use the addon when viewing JSON files
It sadly, doesn't work with an alert, instead it works with a notification (lower right corner)
You can click on the notification to copy the timestamp to your clipboard

- Added an extra option "Decode timestamp (millis)" which will decode an unixtimestamp with milliseconds

Tiny bug fixed, regarding the alert, when the addon is unable to decode the timestamp

Alert added when unable to decode timestamp (eg. if text was selected)
And date will now be written as 01 instead of 1