aggregator: Add a new API to allow selecting start time by subclass

Subclass can have full control of output segment by this new
gst_aggregator_set_start_time_selection_by_subclass() method.
This API would be useful when a subclass has one static input pad
and therefore subclass want to control output segment without
baseclass' help. Just forwarding upstream segment without modification
would be an example.
34 jobs for !597 with live-videorate in 2 minutes and 27 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #4098918
alpine amd64 manifest builder docker

00:00:36

passed #4098917
android docker

00:00:35

passed #4098916
cerbero fedora amd64 docker

00:00:33

passed #4098915
fedora amd64 docker

00:01:52

passed #4098919
gst-indent amd64 docker

00:00:32

 
  Preparation
passed #4098921
gst indent

00:00:34

manual #4098920
manual
manifest
 
  Build
created #4098930
gstreamer
build cerbero cross-android universal
created #4098934
gst-ios-13.2
build cerbero cross-ios universal
created #4098931
gstreamer
build cerbero cross win32
created #4098932
gstreamer
build cerbero cross win64
created #4098929
gstreamer
build cerbero fedora x86_64
created #4098933
gst-macos-10.15
build cerbero macos x86_64
created #4098925
build clang fedora x86_64
created #4098928
1809 windows docker allowed to fail manual
build msys2
created #4098922
build nodebug fedora x86_64
created #4098923
build static fedora x86_64
created #4098924
build static nodebug fedora x86_64
created #4098926
1809 windows docker
build vs2017 amd64
created #4098927
1809 windows docker
build vs2017 x86
 
  Test
created #4098935
gstreamer
check fedora
created #4098936
gstreamer
integration testsuites fedora 1/4
created #4098937
gstreamer
integration testsuites fedora 2/4
created #4098938
gstreamer
integration testsuites fedora 3/4
created #4098939
gstreamer
integration testsuites fedora 4/4
created #4098944
gstreamer
valgrind bad
created #4098941
gstreamer
valgrind base
created #4098940
gstreamer
valgrind core
created #4098945
gstreamer
valgrind ges
created #4098942
gstreamer
valgrind good
created #4098943
gstreamer
valgrind ugly
 
  Integrate
created #4098946
cross-android universal examples
created #4098948
gst-ios-13.2
cross-ios universal examples
created #4098947
documentation