Changes

[GUI.Qt] Some cleaning in qt RealGui (#4641)

* [QtGui] Remove m_createViewersOpt

* [QtGui] Store SofaViewer* directly to avoid multiple dynamic casts

* [QtGui] Remove m_isEmbeddedViewer option to simplify code

* Apply suggestions from code review

Co-authored-by: Frederick Roy <fredroy@users.noreply.github.com>

* Update RealGUI.cpp

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Co-authored-by: Frederick Roy <fredroy@users.noreply.github.com>