doc/manual/dev: single-line comments rule
doc/manual/dev: rules for static, virtual, override, final
AirspaceXSRenderer: Also render "unknown" airspace types

Fixes the display issue of trac ticket #2884.
Manual de: Bulk import of a more recent german manual translation.
manual.mk: Added Claus's beloved index in make rules.
Manual de/ch05_glide_computer.tex: Correct png file name.
Manual images: High resolution topo image.
Manual images: Recent airspace screenshot.
Manual images: Deleted no more used screenshot.