work/cblack/sizegroup

Commit One: Introduce SizeGroup

SizeGroup is a utility component that makes items request the same dimensions.

import QtQuick 2.10
import QtQuick.Controls 2.10
import QtQuick.Layouts 1.10
import org.kde.kirigami 2.14 as Kirigami

Kirigami.ApplicationWindow {
    Kirigami.SizeGroup {
        mode: Kirigami.SizeGroup.Width
        items: [
            boton1,
            boton2,
            boton3,
        ]
    }
    ColumnLayout {
        Button {
            id: boton1
            text: "smol"
        }
        Button {
            id: boton2
            text: "medium"
        }
        Button {
            id: boton3
            text: "superdupermegauberlarge"
        }
    }
}

Commit Two: Make ApplicationHeaders respect size and use SizeGroup

This allows ApplicationHeaders to respect their items' sizes with the help of SizeGroup for keeping all of them the same height.