Tabs default behaviour is synchronous, should be async

[Florian Boucault]

Tabs are often created by developers the following way:

        Tab {
            title: i18n.tr("Simple page")
            page: Page {
                Label {
                    text: "A label"
                }
            }
        }

instead of the lazy loading way that does not penalise app startup:

        Tab {
            id: tab
            title: i18n.tr("Simple page")
            page: Loader {
                parent: tab
                anchors {
                    left: parent.left
                    right: parent.right
                    bottom: parent.bottom
                }
                source: "MyCustomPage.qml"
                active: tabs.selectedTab === tab
            }
        }

This leads to a lot of apps starting slowly. I think we should make the default Tab behaviour to be asynchronous.

[Zsombor Egri (zsombi)]

The Tabs API allows Tab elements to be declared as children. Moving to lazy loading would mean we must have an API break, so all Tabs added will be considered as list<Component>.

[Tim Peeters (tpeeters)]

The Tabs component is being deprecated, but the new AdaptivePageLayout supports asynchronous loading.