Previously, translations were embedded manually, via src/resources.qrc

Now they are compiled and embedded automatically, using embed_translations,
which allows them to be correctly located for multiple targets or archs.

Note that RCC_DIR and LRELEASE_DIR can no longer be specified, because the
system defaults are necessary for multiple targets/archs to build correctly.

The version of Qt must be at least 5.12 for lrelease and embed_translations.

The resources generated by embed_translations include the .qm from
the filename in the resource name, so it is necessary to remove it.
Also, the prefix generated by qmake is /i18n, not /translations.

Now that the .qm files are generated on the fly by the Makefile,
a manual lrelease step is no longer needed and wouldn't work correctly.