STYLE: Simplify error log widget eventFilter

Instead of creating a specific event filter class "qSlicerErrorLogWidgetEventFilter",
re-use the already installed eventFilter associated with the main window.
See "qSlicerAppMainWindow::eventFilter"

See issue #1349

Co-authored-by: Julien Finet <julien.finet@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22305 3bd1e089-480b-0410-8dfb-8563597acbee