bin: Add method to find elements by factory name

A common use case of a dynamically built pipeline is that you want to
(conditionally) find a certain element, e.g. the `rtpbin`s in a
`uridecodebin`. If that element has a fixed name inside its parent bin
(and only has a single instance) this can be easily done by
`gst_bin_get_by_name()`.

If there are multiple instances of the element however, you can only use
`gst_bin_iterate_all_by_interface()`, but this doesn't work if you don't
have the specific `GType` (which is often the case, due to plugins being
dynamically loaded). As such, another fallback could be to use the
well-known name of the element's factory (in case of our example, this
is of course `"rtpbin"`).
27 jobs for wip/nielsdg/bin-get-children-by-type-name in 48 minutes and 51 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #1037225
gst indent

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passed #1037224
manifest

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  Build
passed #1037235
build cerbero cross-android universal

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manual #1037239
gst-ios-13.2 allowed to fail manual
build cerbero cross-ios universal
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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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passed #1037238
gst-macos-10.15
build cerbero macos x86_64

00:07:02

passed #1037230
build clang fedora x86_64

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build fedora x86_64

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1607 windows docker
build msys2

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build nodebug fedora x86_64

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passed #1037228
build static fedora x86_64

00:11:49

passed #1037229
build static nodebug fedora x86_64

00:02:48

passed #1037231
1607 windows docker
build vs2017 amd64

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passed #1037232
1607 windows docker
build vs2017 x86

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  Test
passed #1037240
gstreamer
check fedora

00:04:09

passed #1037241
gstreamer
integration testsuites fedora

00:41:37

passed #1037246
gstreamer
valgrind bad

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gstreamer
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gstreamer
valgrind core

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gstreamer
valgrind ges

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gstreamer
valgrind good

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gstreamer
valgrind ugly

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  Integrate
passed #1037248
cross-android universal examples

00:08:35

manual #1037250
gst-ios-13.2 allowed to fail manual
cross-ios universal examples
passed #1037249
documentation

00:04:39