Support for initial layout in MainView

[glebfox]

In case no view is opened, it is required to display some default layout in the Main View, e.g. with logo or dash board.

---

Let's add initialLayout element to the main view, so if no view is opened, its content is displayed. The logic of handling switching between a view and initial layout must be located in the base class.

Solution

Optional <initialLayout> element is introduced inside <appLayout>, e.g.:

<appLayout>
    ...
    <initialLayout>
        <textField id="textField" label="TextField" clearButtonVisible="true"/>
    </initialLayout>
</appLayout>

---

When added in XML, the root <initialLayout> element effectively is VerticalLayout.

Alternatively StandardMainView's java API can be used to set/get initial layout:

• StandardMainView#getInitialLayout - returns root component of initial layout
• StandardMainView#setInitialLayout - sets a component that will be an initial layout

---

Components inside <initialLayout> can be injected in the MainView, e.g.:

@Route("")
@ViewController("MainView")
@ViewDescriptor("main-view.xml")
public class MainView extends StandardMainView {

    @ViewComponent
    private TypedTextField<String> textField;

    @Subscribe
    public void onInit(final InitEvent event) {
        textField.setValue("Programmatic Value");
        textField.setWidth("25em");
    }

    @Subscribe("textField")
    public void onTextFieldTypedValueChange(final TypedValueChangeEvent<TypedTextField<String>, String> event) {
        Notification.show(event.getValue());
    }
}

[glebfox]

Related Studio bug: https://youtrack.jmix.io/issue/JST-4429/Studio-incorrectly-handles-initialLayout-element-of-MainView