timeoverlay: add a datetime mode

In this mode, buffer timestamps are displayed as an absolute date
since a user-specifiable epoch. The format is also specifiable as
a string property, that will be passed to g_date_time_format().
21 jobs for timeoverlay-datetime-mode in 39 minutes and 40 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Preparation
passed #432999
gst indent

00:00:33

passed #432998
manifest

00:00:29

 
  Build
passed #433005
build cerbero android universal

00:12:40

passed #433006
build cerbero cross win32

00:10:39

passed #433007
build cerbero cross win64

00:08:12

passed #433004
build cerbero fedora x86_64

00:06:07

manual #433009
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #433008
gst-macos-10.14
build cerbero macos x86_64

00:06:44

passed #433000
build fedora x86_64

00:07:18

passed #433001
build nodebug fedora x86_64

00:05:14

passed #433002
build static fedora x86_64

00:13:19

passed #433003
build static nodebug fedora x86_64

00:12:07

 
  Test
passed #433010
check fedora

00:04:38

passed #433011
integration testsuites fedora

00:12:04

passed #433015
valgrind bad

00:06:59

passed #433012
valgrind base

00:16:40

passed #433016
valgrind ges

00:05:05

passed #433013
valgrind good

00:09:38

passed #433014
valgrind ugly

00:02:23

 
  Integrate
passed #433017
android universal examples

00:06:28

passed #433018
documentation

00:05:07