Expose access to GetPropertyChangedSignal

[AmaranthineCodices]

Allows access to property change events using GetPropertyChangedSignal via Roact.Change.[EventName]. This fixes #28.

This will behave like other events; one connection per object, etc.

Note that the vast majority of this is not unit tested. Lemur doesn't implement GetPropertyChangedSignal x.x

Example usage:

local Roact = require(game.ReplicatedStorage.Roact)

local test = Roact.createElement("ScreenGui", {}, {
	Roact.createElement("TextBox", {
		Position = UDim2.new(0.5, 0, 0.5, 0),
		Size = UDim2.new(0, 200, 0, 40),
		[Roact.Change.Text] = function(rbx)
			print("text changed to", rbx.Text)
		end,
		Font = "SourceSans",
		TextSize = 18,
	})
})

Roact.reify(test, game.Players.LocalPlayer:WaitForChild("PlayerGui"))

[coveralls]

Coverage decreased (-3.03%) to 81.765% when pulling 818dd4a on AmaranthineCodices:property-changes into e554ad9 on Roblox:master.

[LPGhatguy]

Ergh. Maybe we should implement support for GetPropertyChanged in Lemur before this lands. I definitely want to improve test coverage as much as possible, which will probably involve rewriting the reconciler tests.

We didn't have test coverage metrics for Lua code when the majority of the Roact tests were written! D:

[AmaranthineCodices]

I would love to get support for GetPropertyChangedSignal in Lemur. I'm not familiar enough with Lemur's codebase and how it does events to implement it though :(

[LPGhatguy]

Stuff is in Lemur!

[AmaranthineCodices]

Cool! I'll try to get to this sometime this weekend.

[AmaranthineCodices]

This was not intended to drag in 27 commits >.<

Going to rewrite the branch history. Again.