Improve Run Configurations

[jankeromnes]

• remove nonofficial run configurations that don't work (e.g. "Run this file")
• hack the "Run" (or "Run Project") button so that it has a better name and becomes a drop-down (instead of running whatever was last or whatever it feels like running for the current file)
• for a workflow item like "Build": "./mach build", repeat the command between brackets in the Run Configuration name like so: Build (./mach build) (to be more explicit, and avoid doing that repetition by hand in janitor.json) (EDIT: Done in Include explicit commands in Run Configuration names. #8)