Add CMake Module for gstapp library

Signed-off-by: Rafał Malinowski <rafal.malinowski@corp.bluecherry.net>
Re-enable rpath for cmake installs

Signed-off-by: Rafał Malinowski <rafal.malinowski@corp.bluecherry.net>
Fix debian files to match new build system

Signed-off-by: Rafał Malinowski <rafal.malinowski@corp.bluecherry.net>