Draft: info: add colord xrandr display id

This MR depends on !110 (merged).

This is a specific display device identification string used with colord to store and fetch information about the device.

Add new API to create a string as specified. New API is required because I want libdisplay-info to reserve the right to change what get_make(), get_model() and get_serial() may return (e.g. prefer DisplayID blob over EDID blob) without breaking configuration keys a user may have stored. When DisplayID becomes supported, the new function may still want to prefer EDID over DisplayID to maintain backward compatibility.

For the above reasons, the implementation does not rely on calling get_make() etc. but depends only on the low-level API.

It is not exactly how Weston implemented this, but the specification is silent about things like using PNP ID or numerical model or serial fields as fallbacks when the names or strings are not available. I'm not at all worried about matching Weston's old implementation, but we need to ensure this is identical to what GNOME uses at least.

Draft, until checked against GNOME.

cc @hughsient