qt: install translations

The Qt translation files are currently created and moved under
$(STAGING_DIR)/usr/translations but not installed on the target.

This patch defines the translations directory to
/usr/share/qt/translations and defines a QT_INSTALL_TARGET_TRANSLATIONS
macro to deploy them from the staging directory to the target directory.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>