Fix WebIO integration, make Window's sync by default.

[twavv]

I think the use of Condition over something like Future from Distributed (builtin package) was a mistake but trying to rectify that wouldn't be worth it right now (and I'm not sure that anyone else will agree with me on that haha).

Anyway, what this diff does:

• Makes Window() sync by default (so that anything you do with the resulting Window is safe).
• Adds Base.wait(::Window). After this returns, the Window should be safe to do things to (e.g. body!).
• Window's that were initialized with a URL have no associated initialization and cannot be wait-ed (I'd rather raise an error that says what you're trying to do doesn't make sense than just silently allowing it).

If you try to do anything before waiting the Window, I make no guarantees and I don't think Blink, in general, should try to do so.

In the future™, we should probably just remove the async keyword in favor of telling the consumer to do windowtask = @async Window().

Ping @NHDaly.

[twavv]

Also: TESTS!!!!! :)))))

[codecov-io]

Codecov Report

Merging #208 into master will increase coverage by 2.43%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   45.27%   47.71%   +2.43%     
==========================================
  Files          12       12              
  Lines         307      306       -1     
==========================================
+ Hits          139      146       +7     
+ Misses        168      160       -8

Impacted Files	Coverage Δ
src/AtomShell/window.jl	38.66% <76.92%> (+3.45%)	⬆️
src/content/api.jl	15% <0%> (+3.88%)	⬆️
src/content/content.jl	100% <0%> (+11.11%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63fcfbb...90af75a. Read the comment docs.

[pfitzseb]

LGTM.

[rdeits]

👍