cmake: Modernize translation generation

Refactor and modernize the CMake support for translation generation.
Properly use targets, custom commands and dependencies to ensure that
files are only regenerated when needed, and avoid useless rebuilds.

Avoid individual invocations of lupdate and lrelease, which both
support processing multiple files at once.

Make use of the newly added quassel_add_resource function instead of
building the .qrc file explicitly.

Fix LINGUAS support, which was previously not working as intended.