* store all qt-only strings in marble_qt.pot, not some in marble.pot
* use $EXTRACT_TR_STRINGS, $XGETTEXT_QT was for kdelibs4-based projects only
* pass UI files directly to $EXTRACT_TR_STRINGS, not via $EXTRACTRC
* be more explicit when finding files to not also include files from
  tools or examples
* create proper QCoreApplication::translate calls for strings extracted
  from data files, when used via Qt translation mechanism.
* separate Messages.sh file for KDE Marble app in its source folder
* use consistently i18n() calls in marble-kde source code

CCMAIL:	kde-i18n-doc@kde.org

Differential Revision: https://phabricator.kde.org/D1763