Add a update-po target in the root Makefile

This just calls the one in the po folder, as a convenience.

It will help for bug #1.
Set the copyright for translations

This seems to be needed for the i18n stuff. (bug #1)
Add the pot template

I just generated it from what we have now, it will most likely change
before long.

This is part of bug #1.
Add the zh_HK.po file

It seems that having a locale in LINGUAS without the corresponding .po
file breaks the build, so let's add it, even if it's completely
untranslated.

However, this finally fixes #1

We can now start accepting translations. :)
Ignore some more built files from po/