A look at terminal emulators, part 1

FYI: profile program execution already existed (it's called custom_command) in Terminator, but didn't work when splitting new tiles (the profile's custom_command was just ignored): this was so inconsistent that many people likely assumed that custom_command it didn't work at all. I fixed this in <https://launchpad.net/bugs/1600609> which has been in terminator's gtk3 branch for a year now.