Image Gallery: Eliminate 'endl' deprecation warnings with Qt 5.15

warning: 'QTextStream& QTextStreamFunctions::endl(QTextStream&)' is deprecated,
use 'Qt::endl'

Redefining the symbol, so as to remain compatible with all Qt versions
(Qt::endl was introduced in Qt 5.14).  'endl' is not needed at the end
of qDebug() output, so it is simply removed.