• Install official PDF User Guide as part of the documentation (version number is hardcoded as it is not always in sync with the PORTVERSION)
• Make telescope control plugin (and Qt5SerialPort dependency) optional
• Remove enforced distfiles checksum recalculation added in r377577: it turned out that updating distinfo manually is easier and a lot faster
• Replace some options imperative (explicit) handling with helper knobs
• Do not install high resolution textures by default, since they do not have their prime meridians recalibrated yet while stock textures have

Discussed with: Alexander Wolf (upstream)