Renamed rest_deps function, and added language parameter.
Made extra dependency system compatible to old extra_deps system.
Added facility to Post object to inject dependencies.
Changed PageCompiler's register_extra_dependencies to get_extra_dependencies, which uses the new utility functions.
Refactored PageCompiler.register_extra_dependencies: split into two functions.
PageCompiler.register_extra_dependencies now emits a warning.
All page compilers in core overwrite this function (so no warning will be displayed).