syntax: Use RGB/GUI attribute information for "abstract_ui"

Instead of using classic cterm color numbers and attributes, treat "abstract_ui"
as a GUI: Pass rgb color numbers and gui attributes when the "highlight_set" UI
method is called. The terminal UI will have to translate RGB color information
to an appropriate color number, and the "term"/"cterm" :highlight keys will
eventually be deprecated.
msgpack-rpc: Add remote_ui module

The remote_ui module is an implementation of the UI layer, and it attaches UI
instances that redirect redraw notifications to connected clients.
test: Add screen test facility implemented as a remote UI