NEWS: prepare for 6.7.3
po: update Russian translation
Makefile, doc: require Android NDK r9c
Manual (en): Flight modes, pages & display modes.
po: update French translation
Manual ui_interface: More over screen pages and the ui concept.
Authors added, sorted alphabetically.
Manual en introduction: Crossref hinting and manual guide.
Manual en navigation: More about auto zoom, and landables in reach.
po: update Japanese translation
po: update Russian translation
Manual en: Started a new quick-start, or hand-out from scratch
taking the goods from the german Blitzeinstieg.
build/manual.mk: Added the english hand-out.
Manual en Flash: Removed dropped in junk from text.
Manual: First step on gathering all the latex macro effort for over all latex docs.
Manual fr: Get the changed macro "bmenug" also into the french manual branch.
build/resource.mk: Fix bmp -> png conversion

ImageMagick 6.8.6 ignored the "png:color-type" ("convert: Cannot write image with
defined png:bit-depth or png:color-type.") and wrote color pngs instead
leading to the assertion:

   xcsoar: src/Screen/OpenGL/UncompressedImage.cpp:70:
     GLTexture* ImportAlphaTexture(const UncompressedImage&):
     Assertion `image.GetFormat() == UncompressedImage::Format::GRAY' failed.

adding "-type GrayScale" fixes the issue.
Manual: Reduce button styles from "de" branch to a more generic set.
Manual en flash: Tiny fix on XC not providing collision info.
po: update French translation
po: update Japanese translation