Greasemonkey: WebExt: Execute scripts at all three runAt times

Come up with a working plan for how to execute user scripts, of course not known until run time. Compatible with all three runAt times -- critically document_start.

See https://github.com/arantius/greasemonkey/tree/8a255bde1aa6312715740fd0157d582a2d6cb183 for my research so far, where the approach is:

• Use one background script. (It will have access to storage/whatever APIs, where the scripts are stored, so can know the content of scripts to execute.)
• Observe something (what?) to trigger ..
• .. call tabs.executeScript() to fire off each user script.

This "works" except there's no event I've yet found to observe that works for all three run times. There's webNavigation's onBeforeNavigate which is in fact too soon, it (as the name implies) fires before the navigation, when the current document is still active. But the next point is onCommitted, and at that point it's too late for start, inline scripts have already executed.