Relative URL support

Except for the first commit, which is a bugfix, this pull request is dedicated to working with URLs and links.

First I replaced `innerHTML` with correct fragment parsing, which takes the base URL into account. Unfortunately, this does work with everything... except links as the special code for opening all links in new tabs broke this functionality. Thus a more simple method of redirecting links was introduced which is by setting the `target` attribute to `_blank` in the `onclick` handler before opening the link.
This new approach used together with automatically focusing the entry link of the selected entry allows to remove intercepting the Enter key and get rid of several special cases _(why if I Enter on a link that opens in a new tab on some webpage the new tab is foreground, but from Brief it used to be a background one?)_ and annoying bugs _(pressing Enter in the address bar, the search bar, a sidebar used to open the selected item - bugs [mozdev#23706](https://www.mozdev.org/bugs/show_bug.cgi?id=23706), [mozdev#24627](https://www.mozdev.org/bugs/show_bug.cgi?id=24627))_.