Adding ability to disable editing node label on double click

[vakokako]

Hi, it would be nice to add support for disabling node label editing on double click.

This implementation adds the enableLabelEditorOnDoubleClick flag to RectNodeTemplate, which allows to override the default behaviour in Node like this:

nodeDelegate: Qan.NodeItem {
    id: nodeItem
    width: 110
    height: 50

    Qan.RectNodeTemplate {
        anchors.fill: parent
        enableLabelEditorOnDoubleClick: false
        nodeItem : parent
    }
}

This is how we implemented it on our side, feel free to suggest another approach.