Usability: change the ADD button's logic to Add / Remove toggle

[Inego]

Currently, if I open a URL from the list and then finish reading it and want to remove from the list, the only option I have is to find the page in the list and click the X button to the left of it, it is not very convenient.
I would like to be able to remove the page from the list by pressing the same button which is used to adding it (after all, in this case "Add" button just does nothing).

[napsternxg]

This is a very useful feature. Would you be able to send a PR for this?

[Inego]

Yes, I will try to.

[Inego]

Here's the pull request.