Using `file(GLOB)` in the generates step is discouraged because the
globbing result may be out of date when the target is built.
Performing the globbing in a script that is executed as the build
target means the result is always reproducable and the overhead
of globbing is only paid when used.
As a follow up, the dependency on `sed` may be removed by performing
the replacement with cmake. Also, the logic from extract_strings_qt.py
can be migrated to cmake.
Daniel Pfeifer