Draft: Modernize debug code:use G_LOG_DOMAIN instead of custom implementation

The previous debug code of EV_DEBUG_JOBS can be taken care of by regular GLib logging. This avoid extra complexity, and makes the debug logging of evince more consistent with everything else. The EV_DEBUG environment variable is dropped in favour of G_MESSAGES_DEBUG="EvJobs EvJobScheduler". The debug border is kept in place, since that does not provide any output, but draws edges around redered entities.

And some extra commits around to simplify the complex one. Depends on !564 (closed) because fixing all the conflicts when merging split would not be fun at all.

I also again really wonder why ev-debug remains in libdocument, when it's actually only used or needed in libview.