st-theme: Use glib auto free/ptr features

Use g_autofree and g_autoptr for managing memory in a smarter way.

GNOME/gnome-shell!536
4 jobs for !536 with st-theme-use-css-instance-data in 17 minutes and 6 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #293661
check_commit_log

00:00:40

 
  Build
passed #293662
build

00:15:02

 
  Test
passed #293663
test

00:01:22

passed #293664
test-pot

00:01:01