docs: use numeric values in enum definitions in public headers

The public headers are parsed with gtk-doc. Define enum values
as plain numbers, otherwise the documentation might have problems
picking up the right value.

#126
5 jobs for th/enum-values-in-api in 58 minutes and 34 seconds (queued for 34 seconds)
Status Job ID Name Coverage
  Test
passed #121682
build_f28

00:33:53

passed #121092
build_f_latest

00:23:33

passed #121093
allowed to fail
build_f_rawhide

00:23:31

passed #121090
allowed to fail
checkpatch

00:01:08

failed #121091
build_f28

00:34:58