Set sourceSize width and height separately

We can't assign a QSize to sourceSize which is a QSizeF so we set
width and height properties separately.
Hide the fill color rectangle during the cross-fade animation

Otherwise it bleeds through and that's ugly.
Comment Background.qml more
Add a resize binding

Signed-off-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Edited-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Support transient and popup surfaces of the panel and background

Signed-off-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Edited-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Remove redundant configure_panel_surface()

We just need one procedure to configure Panel, Launcher and Background
surfaces.
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