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