Make tabs permission optional

[WesleyBranton]

There has been some concern over the addition of the new permission added in pull request #94 for version 3.1.1. To fix the underlying issue, this new tabs permission was added to address issues on older versions. However, it appears that this may not have been the best approach, as noted by the relatively slow adoption rate of this version relative to previous versions. My assumption is that many of the people still on the older version(s) just haven't noticed that they need to grant the new permission unlike the regular automatic updates that happen silently in the background.

Nevertheless, the tabs permission might able to be made optional in a couple of scenarios:

1. The new settings for embedded content are set to "no" by default;
2. The add-on automatically checks if it can access a tab's URL without the permission.

Both options have pros and cons.

Option 1 would be easier to implement, as much of the underlying framework is already in the add-on. Users that change the setting to "yes" would be prompted for the tabs permission to enable the setting. However, the "no" option is the less desirable option and the option that the majority of users likely do not want.

Option 2 would allow most users to have the "yes" option set by default, with only those on older versions of the add-on requiring the default "no" option and needing to grant permission. This has the benefit of only requiring permission from people using browsers that absolutely require that permission to work. The biggest downside to this approach is the increased complexity of the code, the development time required and the increased potential for something to not work properly.

In either case, both options will require a fallback to automatically use the "no" setting if the permission is not granted and/or the browser requires this permission.