darktable 1.6 release candidate 3

this is the third and probably last release candidate before 1.6.0. for almost complete release notes please see the ones for rc1:

https://github.com/darktable-org/darktable/releases/tag/release-1.5.1

if you're a package maintainer, please test this one and let us know if there are any remaining issues with the build or the program.

the checksum is:
sha1sum darktable-1.5.3.tar.xz
4904c4daae62587e3ddda2f5a1d1a5d7fdb1b310 darktable-1.5.3.tar.xz

enjoy!