TabView(Style): Add 'tabsWidth' readonly property

TabView {
    id: tabView
...

    // Workaround to calculate total tabs width
    property Flickable tabRow: findChild(tabView, "tabrow")  // child ListView
    property real tabsWidth: tabRow ? tabRow.contentWidth : 0

    property bool allTabsVisible: width > tabsWidth + 20  // place for '+' button


    function findChild(item, objectName) {
        const children = item.children;
        for (var i = children.length; --i >= 0; ) {
            var child = children[i];
            if (!child.objectName)
                continue;
            if (child.objectName === objectName)
                return child;

            child = findChild(child, objectName);
            if (child)
                return child;
        }
        return null;
    }

    style: TabViewStyle {
...
        tabBar: Rectangle {
...
            Button {
                x: tabsWidth + 1
                visible: allTabsVisible
...

        rightCorner: Row {
...
            Button {
                visible: !allTabsVisible