delay show window
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
Differential Revision: https://phabricator.kde.org/D27170