CMake: Add support for building and installing .mo files

CMake: Add support for building and installing .mo files

Introduce support for building translation domain-specific
templates, merging them with the translations, and building
a language-specific .mo file.

The invocation of xgettext is done in the project source
directory, not in the current source directory, and all paths
are made relative to the project root, in order to have clean
templates.

This only supports the C++ source code for now, it unfortunately
does not handle the shell scripts of deselect yet.