Introduce core properties

[emersion]

This is a set of well-known KMS standard properties, either used
by libliftoff itself or commonly used by compositors. Adding
special cases for these allows us to access them without having
to iterate over a list.

This roughly improves performance by ~50% when leaving out the
atomic test-only commits.

• Consider reserving the first items of the props array for the core props

Closes: #1