Improve markable elements matching

Improve markable elements matching

- Each 'f' command now gets its own set of markable elements. For example, it
  does not make sense to open a button in a new tab.
- The above made it really easy to fix #227: Add 'gf' to follow link in new
  foreground/focused tab.
- Links are now opened in new tabs using Firefox APIs instead of simulating
  ctrl-clicks on the links. This is more reliable (and required in order to
  implement the above point).
- XPath is no longer used. Instead, every element in every frame of the page is
  checked with code. This might sound expensive, but it isn't since that's what
  the XPath matcher needs to do behind the scenes anyway. The benefit of doing
  it with code is that we can do more advanced checks (see below), and no
  information is lost along the way: For example, whether or not an element is
  considered "semantic" is saved, instead of needing to be re-calculated
  afterwards.
- The above fixes #320 in a better way than what was already done in commit
  016574212: Elements that have "button" in their class now must not contain an
  `<a>` or `<button>` element, so a marker for a "button-wrapper" is likely not
  even made and therefore cannot cover the real button inside.
- The above also significantly reduced the number of hints on reddit.com, which
  made the all hints shorter.
- While at it, I implemented #450: 'vf' now selects the contents of text inputs.