Port GObject classes to JS6 classes

GJS added API for defining GObject classes with ES6 class syntax
last cycle, use it to port the remaining Lang.Class classes to
the new syntax.

Fixes #14
1 job for wip/fmuellner/es6-classes in 3 minutes and 27 seconds (queued for 11 minutes and 6 seconds)
Status Name Job ID Coverage
passed build-polari #926