How to use HSplitView to define 3 panes view in SwiftUI for macOS

Issue #674

Specify minWidth to ensure miminum width, and use .layoutPriority(1) for the most important pane.

import SwiftUI

struct MainView: View {
    @EnvironmentObject var store: Store

    var body: some View {
        HSplitView {
            LeftPane()
                .padding()
                .frame(minWidth: 200, maxWidth: 500)
            MiddlePane(store: store)
                .padding()
                .frame(minWidth: 500)
                .layoutPriority(1)
            RightPane()
                .padding()
                .frame(minWidth: 300)
        }
        .background(R.color.background)
    }
}