bus: Use new GSource dispose function

Without this it is possible that we have a GSource with reference count
0 stored in the GstBus that is currently in the process of being
destroyed. gst_bus_remove_watch() might then access it, increase its
reference count to 1 again, call GSource API on it and then unref it,
which will then finalize it a second time.

The dispose function allows the GSource to be resurrected until it
returned so the above would be safe now.

This caused some spurious crashes during shutdown in various
applications.
27 jobs for bus-source-dispose in 55 minutes and 8 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #784609
gst indent

00:00:47

passed #784608
manifest

00:00:32

 
  Build
passed #784619
build cerbero cross-android universal

00:17:16

manual #784623
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #784620
build cerbero cross win32

00:05:20

passed #784621
build cerbero cross win64

00:07:59

passed #784618
build cerbero fedora x86_64

00:05:56

passed #784622
gst-macos-10.14
build cerbero macos x86_64

00:06:47

passed #784614
build clang fedora x86_64

00:08:13

passed #784610
build fedora x86_64

00:06:22

passed #784617
docker windows 1607
build msys2

00:11:27

passed #784611
build nodebug fedora x86_64

00:05:13

passed #784612
build static fedora x86_64

00:13:29

passed #784613
build static nodebug fedora x86_64

00:03:46

passed #784615
docker windows 1607
build vs2017 amd64

00:10:12

passed #784616
docker windows 1607
build vs2017 x86

00:09:43

 
  Test
passed #784624
check fedora

00:04:24

passed #784625
integration testsuites fedora

00:48:10

passed #784630
valgrind bad

00:08:26

passed #784627
valgrind base

00:08:31

passed #784626
valgrind core

00:14:36

passed #784631
valgrind ges

00:05:06

passed #784628
valgrind good

00:10:29

passed #784629
valgrind ugly

00:03:10

 
  Integrate
passed #784632
cross-android universal examples

00:07:14

manual #784634
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #784633
documentation

00:02:36