Refactor and fix wrong usage of UFOReader._readPlist
Refactor and fix wrong usage of UFOReader._readPlist

All cases where UFOReader._readPlist was used suffered
from calling it with the fully constructed path where
it expected just the relative path/fileName of the plist.

Also, all cases had the similiar pattern of checking if
the file exists before trying to read it.

This replaces that pattern by `_getPlist`. The method has
a `default` argument that acts similar to the `default`
argument of the builtin function `getattr` i.e. when
`default is None` the requested plist is mandatory.

fonttools/fonttools,adrientetar/robofab,googlefonts/fonttools