delay show window

    Summary:
    show the window only after mainWidget() of the view has been called.
    mainWidget creates the ui on demand, so at that point the qml scene will be there

    Test Plan: no more empty window for some instants on startup

    Reviewers: #plasma, sitter

    Reviewed By: sitter

    Subscribers: ngraham, sitter, plasma-devel

    Tags: #plasma

    Differential Revision: https://phabricator.kde.org/D27170