gjs 1.52.5 release.

This release includes the "Big Hammer" patch from GNOME 3.30 to reduce memory usage. For more information, read the blog post at https://feaneron.com/2018/04/20/the-infamous-gnome-shell-memory-leak/

It was not originally intended to be backported to GNOME 3.28, but in practice several Linux distributions already backported it, and it has been working well to reduce memory usage, and the bugs have been ironed out of it.

Closed bugs and merge requests:

• Ensure not to miss the force_gc flag
• Make GC much more aggressive
• Queue GC when a GObject reference is toggled down
• Reduce memory overhead of g_object_weak_ref()
• context: Defer and therefore batch forced GC runs
• context: use timeout with seconds to schedule a gc trigger
• Use compacting GC on RSS size growth
• GType memleak fixes