This selects the body of a message, if such is displayed.

1. Use verb phrase for function name.

2. Add `Scroller.reset()` method.  This *only* resets the activated element.

3. Reset the scroller only if the URL has changed.  (Previously, in #3119, the scroller was also being reset when the tab gained the focus.)
   Based on a suggestion from @marcotc.

Twitter is an important site and Vimium's scrolling is currently broken
when a tweet is expanded.

In my opinion, the existing bahviour is so bad and that a site-specific
hack is warranted.

Fixes #3045.

Launching link hints can sometimes be slow.  For filtered hints, the
user already knows what to type - but must nevertheless wait until the
hints have been calculated and rendered.

Here, we cache intervening keydown events, and replay them once
hints-mode proper is launched.  This should make improve usability (in
the case of filtered hints).

Fixes #3050.

First, I *very rarely* use local marks.  They just don't seem as useful
in a browser as they are in a text editor.

However, on a page like GMail, I do often want to jump back and forward
between my labels.  These are different locations hashes (anchors).

The idea here is to make local marks useful for this case.

If the scroll positions (X and Y) are both 0, and the hash (anchor) is
present in the mark and non-empty, then change the hash instead of
scrolling.

On first tests, this appears to work nicely for changing labels in
GMail.  I'm sure there are other uses.

This change shouldn't interfere with use cases where the user has
scrolled within the page.

Chrome has never searched in all frames, so this is no loss.

However, find on Firefox Quantum is now broken with the "all frames"
flag set.

Fixes #2962.

H/t: @gdh1995 (https://github.com/philc/vimium/issues/2962#issuecomment-367991086).