Only export symbols from the libraries that should be exported

Summary:
Using generate_export_header instead manually written export headers
should improve coverage across platforms.

Size of stripped libmarblewidget-qt5 reduced on Linux
e.g. from 11077592 to 7889848

Reviewers: rahn, #marble, nienhueser

Reviewed By: #marble, nienhueser

Differential Revision: https://phabricator.kde.org/D2309