TabBar layout ignores vertical separators width

[stefanomondino]

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• Updated Tabman to the latest version
• I took a look at the Troubleshooting Guide
• I searched for existing GitHub issues

Issue Description

When using separators in TabBar (TMConstrainedHorizontalBarLayout), the inner stack view is applying a width constraint on both items and separators, ignoring the separator's width.

This results in button items and separators having the same width, which is not the intended behavior.

This can easily be reproduced by changing provided example with this snippet in TabViewController.swift instead of ButtonBar in viewDidLoad

 // Create a bar
        let bar = TMBarView.TabBar()
        
        // Customize bar properties including layout and other styling.
        bar.layout.contentInset = UIEdgeInsets(top: 0.0, left: 16.0, bottom: 4.0, right: 16.0)
//        bar.layout.interButtonSpacing = 24.0
//        bar.indicator.weight = .light
//        bar.indicator.cornerStyle = .eliptical
        bar.fadesContentEdges = true
        bar.spacing = 16.0
        
        bar.layout.separatorWidth = 1
        bar.layout.showSeparators = true

    bar.buttons.customize {
            $0.tintColor = UIColor.tabmanForeground.withAlphaComponent(0.4)
            $0.selectedTintColor = .tabmanForeground
            if #available(iOS 11, *) {
                $0.adjustsFontForContentSizeCategory = true
            }
        }
        bar.indicator.tintColor = .tabmanForeground
        
        // Add bar to the view - as a .systemBar() to add UIKit style system background views.
        addBar(bar.systemBar(), dataSource: self, at: .top)

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