[WIP] Basic Edge support

[mrmr1993]

Currently, this gives basic support to Edge (issue philc#2076). Most of the changes are fairly unobjectionable, 961863e and 30a36d6 being the notable exceptions.

Things that I've tested and found not working:

• Visual modes, due to no support for the non-standard Selection.modify that we use.
• focusInput, where Edge rejects the XPath that we use to get the input elements
• simulated clicks hit the pop-up blocker (as in Firefox, so we can use the same workaround), so some link hint commands don't work
• clipboard commands (esp. copy) are unreliable
• all commands/UI using UIComponent -- the HUD, Vomnibar and the help dialog -- are broken
• exclusions pop-up is partly concealed by a scrollbar (at least, in my Window VM)

Edit (2017-11-15):

• UIComponent frames don't get focused when activated
• multi-frame link hints don't work properly/reliably
• find mode doesn't work
• Vomnibar doesn't seem to get suggestions from history