CMake: Translations: Add support for shell scripts

CMake: Translations: Add support for shell scripts

Rework the arguments to apt_add_translation_domain so a user
can specify TARGETS and SCRIPTS, the latter being Shell scripts.

For each language (TARGETS being C++, SCRIPTS being Shell), a separate
template is generated via xgettext. Those templates are then merged
together by using msgcomm. In case there are no Shell scripts in
the translation domain, msgcomm will receive /dev/null instead of
a shell translation template.

This also reintroduces line numbers, as msgcomm would otherwise
re-order the merged files not only by filename, but also by message
string. It's unclear why it does that, it could just leave strings
within a file alone.

In contrast to the old build system, we use xgettext for shell scripts
instead of bash --dump-strings, as it's just easier to use the same
tool for everything. We also create valid headers.