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This is a major reworking of how the search-completion systems works.
Previously, the goal had been to emulate roughly how chrome itself
handles completions.  This has turned out to be a bad idea:
   - It creates issues around what it means to hit <Enter> in the
     vomnibar.
   - And the contents of the vomnibar change asynchronously (because we
     fetch completions asynchronously), so it creates the possibility
     that the effect of <Enter> changes depending on how long the user
     waits before typing <Enter>.
All of that is bad.
This commit changes things:
   - In normal omni mode, the vomnibar looks and behaves pretty much
     like it always has, just with some extra completion suggestions
     thrown in.
   - And in custom-search-engine mode it also behaves mostly as it has
     previously, but (again, possibly) with some extra completion
     suggestions thrown in, and with many useless suggestions excluded.
This is all far more Vimium-like than Chrome-like.
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