getViewportTopLeft: support zoomed static documentElement

If an element's `contain` is/contains `paint`, then it will be forced
showing just as its `position` is `relative`.

if `document.documentElement` is zoomed, Vimium's hints `<div>` are also zoomed,
and then `scrollY` may be not equal with viewport client rect's `top`.

Example:
* make the tab zoom level is 1, `documentElement`'s `zoom` style is 2
* open a page, scroll to the top left corner
* press `f`, and all things are right
* exit LinkHints mode, scroll down for 100px, and then press `f`
* this time all hints has moved down 200px from the correct place

New command option to enter normal mode

`SearchEngines` was previously in `utils.coffee`, which means it was
loaded in *every* content frame.  This is unnecessary, since it is only
used on the background page.  So this PR moves it there.

Also:

- Simplify some unnecessarily complex logic in `vomnibar.coffee`.
- Re-use `Utils.parseLines()` to parse the custom search engine configuation text.

Here's the problem...

Many sites define their own keyboard shortcuts, for example Google Play
Music defines `gh` for "go home".

On such sites, it's natural to set up pass keys for `g` and `h`.

But that makes any Vimium key bindings which begin with `g`
inaccessible.

Here, we add a new command `enterNormalMode` which installs a new
normal-mode instance (without any pass keys).  This executes a single
normal-mode command then exits.

Example:

    map \ enterNormalMode
    map | enterNormalMode count=999999

Assuming `g` and `o` are pass keys:

- `gh` or `o` - use the page's binding
- `\gg` - scroll to top
- `2\ggo` - scroll to the top and open the Vomnibar
- `\g<Escape>o` - open the Vomnibar
- `\<Escape>o` - use the page's bindings
- `\\\\\\<Escape>o` - use the page's bindings (new normal-mode instances
  displace previous ones)

This required some changes to the scroller.  Previously, we only ever
had one normal-mode instance, and could arrange statically that the
scroller's key listeners were above normal-mode's key listeners in the
handler stack.  Here, we fix this by adding and removing the scroller's
listeners dynamically, so they're always at the top of the handler
stack.