First round of patches to make it easier to work with the MainWindow

[Tomaz Canabrava]

this series of patches reworks quite a lot of the way we deal with the
mainwindow internally. the idea is to make things more consistent when
changing widgets around.

We used  to have a very complex layout of different stack widgets and
indexes represeenting states of the application,

now we need to:

registerApplicationState( name, top-left-widget, bottom-left-widget,
top-right-widget, bottom-right-widget )

then

setApplicationState( name )

when we need to set the state.
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