Add Screen.ChannelEvents v2

[eNV25]

This PR proposes an alternative API for getting Events. Version 2. The following method is added:

type Screen interface {
	...
	// ChannelEvents is an infinite loop that waits for an event and
	// channels it into the user provided channel. When Fini() is called
	// or quit is closed, the function returns. This function should be
	// used as a goroutine.
	ChannelEvents(ch chan<- Event, quit <-chan struct{})
	...
}

Previous version: #461.

[codecov]

Codecov Report

Merging #465 (6224fe8) into master (da8f206) will decrease coverage by 0.65%.
The diff coverage is 25.92%.

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
- Coverage   16.10%   15.44%   -0.66%     
==========================================
  Files          21       22       +1     
  Lines        1472     1580     +108     
==========================================
+ Hits          237      244       +7     
- Misses       1221     1322     +101     
  Partials       14       14              

Impacted Files	Coverage Δ
screen.go	0.00% <ø> (ø)
tscreen.go	0.00% <0.00%> (ø)
simulation.go	53.84% <53.84%> (ø)
tty_unix.go	0.00% <0.00%> (ø)
nonblock_unix.go	0.00% <0.00%> (ø)
stdin_unix.go	0.00% <0.00%> (ø)

---

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[eNV25]

I have simplified the implementation from before, and I have added tests too.

Now the only potential problem is what happens when you call PollEvent while ChannelEvents is running.

[gdamore]

I wouldn't worry about that problem. We should just document that two are incompatible.

[eNV25]

I don't understand how just changing a doc comment reduces test coverage.
https://app.codecov.io/gh/gdamore/tcell/compare/465/commits

[gdamore]

Thanks for your contribution!