user-accounts: Use g_signal_connect_object with UmUserManager

The UmUserManager object could last longer than the panel. Use
g_signal_connect_object to do this automatically. Remove manual old method.

Connect signals in swapped form.
11 jobs for users-connect-object in 3 minutes and 49 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #130504
build

00:02:00

 
  Test
passed #130506
asan

00:01:43

passed #130505
test

00:01:49

 
  Manual
manual #130511
allowed to fail manual
armv8
manual #130507
allowed to fail manual
flatpak
manual #130510
allowed to fail manual
msan
manual #130512
allowed to fail manual
ppc64le
manual #130508
allowed to fail manual
tsan
manual #130509
allowed to fail manual
ubsan
 
  Review
manual #130513
allowed to fail manual
review
manual #130514
allowed to fail manual
stop_review