Support for translations with bundled apps, for now Marble Maps

Summary:
A new Script download-pos.sh allows to get marble_qt po files either
for trunk, stable or a tag, either all or a certain language.
Example: data/lang/download-pos.sh trunk de
To be used optionally with packaging or development builds.

Additionally a target "bundle_translations" is added which can
be explicitly invoked to generate qm files for the po files
fetched before with download-pos.sh

Also remove outdated scripts from qt4 times.

Reviewers: shentey, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D3094