region: Replace old style packing properties with GtkWidget properties

Note that the toolbar properties can't yet be changed:
GNOME/gtk!334
11 jobs for region-packing in 4 minutes and 34 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #106996
build

00:02:16

 
  Test
passed #106998
asan

00:02:16

passed #106997
test

00:00:43

 
  Manual
manual #107003
allowed to fail manual
armv8
manual #106999
allowed to fail manual
flatpak
manual #107002
allowed to fail manual
msan
manual #107004
allowed to fail manual
ppc64le
manual #107000
allowed to fail manual
tsan
manual #107001
allowed to fail manual
ubsan
 
  Review
manual #107005
allowed to fail manual
review
manual #107006
allowed to fail manual
stop_review