Qt'fy access to pimpl objects: use macros Q_DECLARE_PRIVATE & Q_D

Summary:
* enforces constness some more
  (see issue spotted with GeoDataTrack::extendedData())
* delivers inline-ness of the access methods
* code should be more familiar to Qt developers

Reviewers: #marble, nienhueser

Reviewed By: #marble, nienhueser

Subscribers: nienhueser

Differential Revision: https://phabricator.kde.org/D2951