* New upstream stable release (THUNDERBIRD_52_1_0_BUILD2)
  - see USN-3278-1
* Update and reenable unity-menubar.patch
* Refresh patches:
  - update debian/patches/support-coinstallable-trunk-build.patch
* Ensure we generate all of the required checksum files for FIPS mode
  to work correctly
  - update debian/build/rukes.mk
* Unconditionally build-dep on libffi-dev
* Refresh patches
  - update debian/patches/support-coinstallable-trunk-build.patch
  - update debian/patches/unity-menubar.patch
* Pass "-Wnull-dereference -fno-delete-null-pointer-checks" as workaround
  for compilation with GCC6
  See https://bugzilla.mozilla.org/show_bug.cgi?id=1251576
* Add build-dep on libx11-xcb-dev
* Install the gtk2 directory and ICU data file
  - update debian/thunderbird.install.in