This makes global marks use the same code for opening new tabs as other
operations.  Thus, the new tab appears in the same position as it would
if it were created, for example, via the vomnibar.

To do this properly, we move the three tab-creating functions into an
object which can be (and is) exported (TabOperations).

Also fixes these three operations such that they all take the same
arguments; that is (request, callback).  Not all of these callbacks are
being used.  But we should be consistent.

If the tab is subsequently deleted, then we return to the original tab
(as opposed to the one on its right).

... as suggested by @mrmr1993 in #1728.

Note. This does not allow tabs to rotate from the left around to the
right, or vice versa.  Which means "999<<" moves the current tab all the
way to the left (and similarly to the right).

Fixes #1727 (kind of).

With #1697, the vomnibar used with a custom-search keyword is not
initially empty (it shows the primary suggestion for the custom search
engine, even if the query is empty).  The way things were structured
previously, the user nevertheless had to wait until the history cache
(which was not actually required) had been fetched before the vomnibar
was updated.

This commit just flips things around a bit such that,
   - onComplete() is called immediately if the history is not actually
     required, and
   - the history cache is primed before it is needed, so it will
     (hopefully) be available before the user's next keystroke.

This avoids a noticable delay, particularly on start up and with a large
history.

(This is part of a sequence of vomnibar UX tweaks.)

Conflicts:
	background_scripts/completion.coffee
	background_scripts/completion_engines.coffee

On the help page, use separate row for very-long bindings.

When filter suggestions from other completers, most notably the history
completer, we only keep suggestions which match the current searchUrl
and completer.  Here, we also *replace* the URL of the suggestion.  With
duplicate elimination, multiple history entries (e.g. those generated
with various "Search Tools" settings on Google) will be collapsed to
one.  This matters because, with custom search engines, we don't show
the URL, so the user can't see differences in the URL.  Without this,
the user can be presented with a list of apparently identical
completions.