GObject: Add convenience wrapper for signal_handler_(un)block()

Just like the existing disconnect(), those functions are more
idiomatic as object methods than the underlying static functions.

GNOME/gjs!326
35 jobs for block-handler in 37 minutes and 34 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Source Check
passed #390712
cppcheck

00:01:11

passed #390544
cpplint

00:02:29

passed #390545
eslint

00:01:23

passed #390542
fedora

00:16:28

passed #390709
cppcheck

00:00:50

failed #390672
cppcheck

00:00:36

failed #390543
cppcheck

00:01:26

 
  Test
passed #390755
no_graphics

00:02:15

passed #390758
no_profiler

00:02:19

passed #390551
no_readline

00:04:04

passed #390763
sanitizer_gcc

00:02:51

passed #390780
with_logging

00:03:15

passed #390788
with_systemtap

00:02:28

canceled #390549
no_graphics

01:01:31

canceled #390550
no_profiler

00:38:32

canceled #390546
sanitizer_gcc

00:51:07

canceled #390547
with_logging

00:51:05

failed #390786
with_systemtap

00:00:53

canceled #390548
with_systemtap

00:51:02

 
  Manual
manual #390565
allowed to fail manual
armv8
manual #390552
allowed to fail manual
codequality
manual #390553
allowed to fail manual
code_statistics
manual #390567
allowed to fail manual
coverage
manual #390555
allowed to fail manual
fedora_gcc
manual #390561
allowed to fail manual
flatpak
manual #390556
allowed to fail manual
installed_tests
manual #390559
allowed to fail manual
lts
manual #390566
allowed to fail manual
ppc64le
manual #390554
allowed to fail manual
sanitizer_clang
manual #390558
allowed to fail manual
ubuntu_clang
manual #390557
allowed to fail manual
ubuntu_gcc
manual #390560
allowed to fail manual
valgrind
manual #390564
allowed to fail manual
zeal_11
manual #390562
allowed to fail manual
zeal_2
manual #390563
allowed to fail manual
zeal_4