simplifies the mainwindow ui stacked widget
removes the progamatic construction and replaces it
with the designer QML.
Also factorises the code of slidepage in Utils.h for
a consistent page switching at all levels of UI.

Change-Id: I75ee29b0b93de63978262db4da04dc6c96e0942e
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>