* Devtools device description: fix the case of the 'platformversion' property
* Devtools device description: expose the properties Firefox expects for the display name and version of the server
When connecting to Servo from Firefox (about:debugging), it will display "Servo (71.0)" instead of "{$name} (${version})".
* Devtools device description: update versions to match those in the UA string
* Devtools device description: expose a property (appbuildid) Firefox expects to compare versions for compatibility (_compareVersionCompatibility function)
* Devtools device description: advertise a current platform version to remove the incompatibility warning in Firefox's about:debugging
* Devtools preference actor: fix retrieving the actual preference by its key
* Devtools preference actor: translate Firefox preference names onto the corresponding Servo preference names
* Devtools device description: downgrade the platform version to prevent a confusing warning in about:debugging if the build ID is newer than that of Firefox (likely)
* Devtools device description: add comments
* Devtools device description: use CARGO_PKG_VERSION for the user-visible version number
