[glyphs] Handle additional custom params

[cmyr]

This is based on #1143

This handles additional custom parameters, specifically openTypeHeadLowestRecPPEM, and the various hhea & vhea caret properties (vhea isn't handled yet, but we can at least parse it now in preparation).

These params are kind of funny because you don't seem to be able to add them via the glyphs.app UI? But they exist in some fonts, and were causing crater failures.

• closes handle openTypeLowestRecPPEM value in glyphs sources #1128

[khaledhosny]

These params are kind of funny because you don't seem to be able to add them via the glyphs.app UI?

You can add anything as Glyphs custom parameter. If Glyphs recognizes the name, it will give it the appropriate UI (e.g. a check box for boolean parameters), but otherwise you get a default text entry box that can have any value. Some of the parameters that Glyphs recognizes are hidden; if you search for them they will not show up, but if you go ahead and add it anyway it will get the appropriate UI.

They openType parameters, are, I think, present to parallel the corresponding UFO fontinfo keys, and I think Glyphs will use them when importing UFO files, rather than the “native” equivalent custom parameters.