combine_join block

[MayaGans]

Changelog

1. UI of join block is now an E that accepts two data frames or their output:
2. UI of data blocks now have connection to be included within a join block:

Does this look like what you were envisioning, @gvwilson ? I wonder if there's a way to validate that the first connection [here: colors] is of type data (or maybe that'd be too restricting?)