Allow assignments with property paths to use x.y.z = value syntax

Allow assignments with property paths to use x.y.z = value syntax

This is actually an embarrassing oversight in issue 2883: while it
became possible to rewrite

    markup-system-spacing #'minimum-distance = #25
as
    markup-system-spacing minimum-distance = #25

it has been overlooked to allow for the obvious

    markup-system-spacing.minimum-distance = #25


I am tempted to sneak this into 2.18.0 yet since this is

a) trivial
b) will match user expectations

Of course, the current time frame would not allow regtests and usage
examples to be added: this would simply be provided in order to avoid
surprises to people who assume this should work.