Filter hidden tabs out when counting them

[vhfmag]

This proposed PR tries to solve #22 in the simplest way possible (ignoring hidden tabs) but it's worth evaluating if there is value in adding a new item in the popup for counting them or adding an option to ignore them, etc.

Feedbacks are welcome and thanks for this extension!

Closes #22

[vhfmag]

For anyone following this, I have published this fork, that addresses #22 and #24, closing this PR and #26.

[DaAwesomeP]

@vhfmag Thank you for submitting pull requests. I apologize for my inability to maintain this repo. It is very difficult to dedicate time to develop and test something like this amidst a very busy schedule and many projects. Thank you for understanding that this project is not my highest priority, but I am very happy for its development to continue. Pull requests are require significantly less work on my end to push through. I really appreciate it. 6,000 people use this addon, and I'm sure most people notice these issues. I do not want to deprecate this repo because of how many people use the addon, but people like you allow it to stay alive.

Because this is open source code, you are free to do whatever you like with it (fork it, publish it, etc.) so long as you follow the Apache 2.0 guidelines (provide a copy of the license, attribution, etc.). This is the beauty of open source code!

I will accept this pull if you set the minimum FF version in the manifest to be the first version that FF introduced this option so that earlier FF versions aren't upgraded and won't throw errors.

Also please give it a whirl in Opera and Chrome. If this feature doesn't work/isn't implemented in those, you'll have to factor in more if statements. I will push my progress on this last time I had a chance to work on it to another branch.

[DaAwesomeP]

OK, I pushed 71e595b, which shows the version checking I was getting at. It looks like I was also going to implement an option to count hidden tabs, but anything is good at this point!

[vhfmag]

@DaAwesomeP nothing to apologize about. Free labour is free and, therefore, optional, and you should be doing whatever you want with your life. Thanks for your contribution, I don't think I'd have ever built such an extension myself. I intended to be more polite and be explicit about letting you know this isn't a rage fork, but simply a away to get an extension with those PRs merged into the store so that I could get rid of the annoying warning about it not being signed.

That said, I will try to do the suggested changes when I can.

Again, thank you!