ENH: Consolidate qSlicerAppMainWindow

qSlicerAppMainWindowCore brings confusion to the code. It is now merged
with qSlicerAppMainWindow.
The custom macro is replaced by the automatic signal/slot connection of Qt
i.e. on_<SENDER_NAME>_<SENDER_SIGNAL>())
The objects had to be renamed to support the feature.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Julien Finet <julien.finet@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22763 3bd1e089-480b-0410-8dfb-8563597acbee