Save Tabs to JSON wót Eztaban

Will go through all open tabs in current or all windows and store them in a JSON file where the key is "urls" and the value is a list of URLs.
The extension can also load a JSON file of same format and will open all URLs in a separate tab.
To load a file with URLs, a new tab will be opened, prompting the user to select a file. This opens a dialog to the local filesystem, where the user can select a JSON file of the same structure as described above.
Each URL will be opened in a separate tab. The tab opened for file selection will be closed at the end.

Tabs stored from separate windows will be opened in separate windows again.
Tabs stored from a private or public window will be opened in private/public window again.