grc: add domain xml DTD and parsing
grc: add domain attribute to ports
grc: draw ports with custom domain differently
grc: don't fail for unknown domains
grc: use domain connection templates in Generator
grc: domain-specific port keys
grc: per element line styles (for custom domain ports and message connections)
grc: add gr_message domain
grc: make Generator use gr_message domain
grc: add multiple_sources flag to domain.dtd
grc: PEP8 fixes in Generator
grc: port connections by domain, block name in generated code
grc: add domain property color and use it for connections
grc: fix connections error log and color
grc: port domain code clean-up
Merge branch 'port_domains' into master_grcwg