Support resizing the main pane in custom layouts

[adambossy]

Is your feature request related to a problem? Please describe.
I've created a custom layout that's similar to the 3-Column Middle layout that ships with Amethyst, except that it lays out secondary windows as columns instead of rows on either side of the main, central pane. I would like to support resizing the main pane in the same way I can the 3-Column Middle layout.

It seems that the way this is done in ThreeColumnLayout.swift is by reading mainPaneRatio from the decoder that's passed to the constructor, but this does not seem to be supported in the custom layout API.

Describe the solution you'd like
I would like to be able to implement the equivalent of this Swift code in a JavaScript layout extension in order to support main pane resizing:

    required init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        self.mainPaneCount = try values.decode(Int.self, forKey: .mainPaneCount)
        self.mainPaneRatio = try values.decode(CGFloat.self, forKey: .mainPaneRatio)
        super.init()
    }

Describe alternatives you've considered
I've looked into whether updateState or any other methods could support my desired behavior but it's not clear that they'd work.