Request tabs Permission Only When Needed

[rthaut]

This is related to #50

Since the tabs permission actually provides a lot of power, and since the wording for both Firefox and Chrome (but mostly Chrome) is very misleading, it would probably be better to only request the tabs permission when the "Close Original Window/Tab" setting is enabled.

Furthermore, since that setting is disabled by default, its possible that many users won't even enable it, so always requiring the tabs permission then becomes careless.

Per the documentation:

requests may only be made in the handler for a user action (for example, a click handler)

With that in mind, the most logical way of handling this is to request the tabs permission when the user toggles the "Close Original Window/Tab" setting to "on," and it would also be logical to remove the tabs permission whenever that setting is toggled back to "off."