st/label: Compare shadow spec before invalidating shadow on style change

Just like with did with last commit for StEntry, check whether the text
shadow actually changed before invalidating the pipeline on style
changes of StLabel.
11 jobs for !1744 with st-entry-and-label-style-changed in 6 minutes and 2 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Pre Review
passed #1189923
check_commit_log

00:00:27

passed #1189924
check-merge-request

00:00:16

 
  Prep
passed #1189925
build-fedora-container

00:00:45

 
  Review
passed #1189927
eslint

00:00:54

passed #1189928
eslint_mr

00:00:51

passed #1189926
js_check

00:00:28

passed #1189930
no_template_check

00:00:22

passed #1189929
potfile_check

00:00:16

 
  Build
passed #1189931
build

00:04:33

passed #1189932
flatpak
flatpak

00:02:11

 
  Test
passed #1189933
test

00:01:01