Add engine.gettext() and remove gettext() calls in guiFormspecMenu.cpp

This solves two issues at once:
* CMake would delete po files during ‘make clean’ because it thought
  they were autogenerated and not just managed
* the only gettext tools readily available in Windows are so old they
  don't support options like --package-name

The change also moves minetest.pot down one level, so we don't need to
special case ‘en’ anymore.

The downside is of course that you need some sane POSIX shell to update
the po files.