Add support for sources reverse playback

It introduces a `reverse` property at the `nlesource` level which will
basically reverse playback of the clip.

We guarantee that outside that clip, the data flow looks like it was
playing forward as we add an 'identity' element with `single-segment=true`.

This start making use of the newly introduced concept of
`CHILD_PROP_FLAG_SET_ON_ALL_INSTANCES` as it is natural that
doing `source_clip.set_child_property("reverse", True)` will make
the whole clip go reverse, meaning that all the GESSource children
properties will be set. To set an individual source "reverse" child
property as the desired effect.

Depends on: gstreamer/gstreamer!451
24 jobs for !188 with reverse_playback in 15 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Preparation
passed #3028545
gst indent

00:00:15

manual #3028544
manual
manifest
 
  Build
created #3028554
gstreamer
build cerbero cross-android universal
created #3028558
gst-ios-13.2
build cerbero cross-ios universal
created #3028555
gstreamer
build cerbero cross win32
created #3028556
gstreamer
build cerbero cross win64
created #3028553
gstreamer
build cerbero fedora x86_64
created #3028557
gst-macos-10.15
build cerbero macos x86_64
created #3028549
build clang fedora x86_64
created #3028552
docker windows 1809 allowed to fail manual
build msys2
created #3028546
build nodebug fedora x86_64
created #3028547
build static fedora x86_64
created #3028548
build static nodebug fedora x86_64
created #3028550
docker windows 1809
build vs2017 amd64
created #3028551
docker windows 1809
build vs2017 x86
 
  Test
created #3028559
gstreamer
check fedora
created #3028560
gstreamer
integration testsuites fedora 1/4
created #3028561
gstreamer
integration testsuites fedora 2/4
created #3028562
gstreamer
integration testsuites fedora 3/4
created #3028563
gstreamer
integration testsuites fedora 4/4
created #3028564
gstreamer
valgrind ges
 
  Integrate
created #3028565
cross-android universal examples
created #3028567
gst-ios-13.2
cross-ios universal examples
created #3028566
documentation