notifications: Use GtkTemplate

Switch from GtkBuilder to using GtkTemplate.
Rename widget IDs to be more readable.
Drop widget IDs that are not used.
Move code into the .ui file that can be.
Connect signals in swapped form.
11 jobs for notifications-gtktemplate in 6 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #126622
build

00:01:59

 
  Test
passed #126624
asan

00:02:14

passed #126623
test

00:01:54

 
  Manual
manual #126629
allowed to fail manual
armv8
manual #126625
allowed to fail manual
flatpak
manual #126628
allowed to fail manual
msan
manual #126630
allowed to fail manual
ppc64le
manual #126626
allowed to fail manual
tsan
manual #126627
allowed to fail manual
ubsan
 
  Review
manual #126631
allowed to fail manual
review
manual #126632
allowed to fail manual
stop_review