Fix parsing of geo coordinates in locale encoding (+ extend unit test)

Summary:
QString::toDouble in Qt5 no longer first tries the system locale

In tests also prefer QLocale::toString over "%L1", in some Qt versionns
(e.g. 5.6) the L might only work when QLocale::setDefault has been
explicitely called.

Test Plan:
Unit tests haven been improved and are all passing.
marble --latlon "46,09898 7,78307" in German locale works now as expected

Reviewers: rahn, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D1994