st-theme: Use saner styleshet refcounting and auto-cleanup function

CRStyleSheet refcounting starts at 0, this makes handing it in a bit
different way, compared to how we're used with GObject.

In order to use the same paradigm everywhere, add a reference to all the
stylesheet we create and we manage the memory, so that we can use them in
the same way we'd do with other objects.

Add an autoptr definition so that we can use smart pointers to handle the
stylesheets lifecycle easily.

GNOME/gnome-shell!536
9 jobs for !536 with st-theme-use-css-instance-data in 6 minutes and 22 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #817688
check_commit_log

00:00:17

passed #817690
eslint

00:01:26

passed #817689
js_check

00:00:42

passed #817692
no_template_check

00:00:31

passed #817691
potfile_check

00:00:48

 
  Build
passed #817693
build

00:03:42

passed #817694
flatpak
flatpak

00:02:33

 
  Test
passed #817695
test

00:00:50

passed #817696
test-pot

00:01:11