MFEKglif must preserve point `smooth` in .glif files and allow users input into its value

[ctrlcctrlv]

Based on Twitter discussions. This is a tracking issue for—

https://twitter.com/MFEKglif/status/1476756164478128140

Next month (next year heh) we'll update MFEKglif to respect the UFO glif "smooth" property, also we will only remove a "smooth" in a file we're loading in if it's obviously invalid (the point claims to be smooth but the control points aren't even linear within 1 unit so it isn't)

https://twitter.com/MFEKglif/status/1476756918966587396

We'll also let users make points smooth/tangent in the editor itself, and we'll use that as part of determining whether to force a line, instead of Shift key, as recommended by @jenskutilek and @simoncozens unless the user explicitly says not to.