signal-desktop: 1.18.1 -> 1.19.0

A new dependency (at-spi2-atk) is required for libatk-bridge-2.0.so.

Also libstdc++.so.6 is now required from stdenv.cc.cc.lib (stdenv.cc.cc
doesn't offer it). The binary will only find libstdc++.so.6 via
$LD_LIBRARY_PATH (i.e. not via rpath, the run-time search path).