Expose GetPropertyChangedSignal declaratively

[LPGhatguy]

Right now, you can connect to the Changed event in Roact easily with Roact.Event.Changed, but using GetPropertyChangedSignal(name) on a specific property is much more performant and is much cleaner.

Roact makes that painful right now -- the only way right now is to connect in didMount and disconnect in willUnmount, typical for wrappers around APIs like this.

I think we can make Roact support GetPropertyChangedSignal as a first-class citizen for events.

[AmaranthineCodices]

Proposed API:

Roact.Change.[PropName]

e.g. for a TextBox:

Roact.createElement("TextBox", {
    [Roact.Change.Text] = function(rbx)
        print("text changed to "..rbx.Text)
    end,
})

[AmaranthineCodices]

This can be closed now after #51 - I forgot to mention this issue in that PR!

[LPGhatguy]

This is super super handy, thanks!