Composite UI components may not handle UI events

[glebfox]

Relates to #400

CompositeComponent handles UI events only if composition is set using the setComposition() method. If we define root directly, events are not handled.

Steps to reproduce

Take the test CompositeComponent from the linked issue and replace

protected void onCreate(CreateEvent event) {
    CssLayout layout = uiComponents.create(CssLayout.NAME);
    setComposition(layout);
}

with

protected void onCreate(CreateEvent event) {
    root = uiComponents.create(CssLayout.NAME);
}

AR:

UIRefreshEventwon't be handled