Fix tab focus in the widgets and QML webviews

Fixes:
- Make sure that we call SetInitialFocus when giving focus through Tab
  This does the same as would WebContents::FocusThroughTabTraversal
- Implement QWebEnginePagePrivate::passOnFocus
- Set each new RWHVQtDelegate as the focus proxy of the QWebEngineView
- Make sure that the widgets delegate accepts the tab focus policy

Cleaups:
- RenderWidgetHostViewQtDelegateQuick doesn't need to be a focus scope,
  it doesn't have any children
- We don't need to reimplement QQuickWebEngineView::forceActiveFocus
  since the view is now a focus scope
- Do not explicitly setFocus(true) on the QQuickWebEngineView,
  the application should decide this through the API

Change-Id: I817dc2c895d4fff4aa3536c71ecc5d306bb3bee0
Reviewed-by: Andras Becsi <andras.becsi@digia.com>