To extract Qt tr translatable strings from C++ files for Qt5-based projects
$EXTRACT_TR_STRINGS needs to be used, $XGETTEXT_QT has been for kdelibs4-based projects only.
See also https://techbase.kde.org/Development/Tutorials/Localization/i18n_Build_Systems#Writing_a_Messages.sh_script
UI files are passed 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.
Have separate Messages.sh file for KDE Marble apps in its source folder.
Use consistently i18n() calls in marble-kde source code.