timeoverlay: add a property to show times as dates

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 83 minutes and 21 seconds (queued for 51 seconds)
Status Job ID Name Coverage
  Preparation
passed #474279
gst indent

00:00:28

passed #474278
manifest

00:00:31

 
  Build
passed #474285
build cerbero android universal

00:12:26

passed #474286
build cerbero cross win32

00:09:28

passed #474287
build cerbero cross win64

00:06:10

passed #474284
build cerbero fedora x86_64

00:06:47

manual #474289
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #474288
gst-macos-10.14
build cerbero macos x86_64

00:06:49

passed #474280
build fedora x86_64

00:10:15

passed #474281
build nodebug fedora x86_64

00:07:30

passed #474282
build static fedora x86_64

00:11:26

passed #474283
build static nodebug fedora x86_64

00:14:40

 
  Test
passed #474290
check fedora

00:05:23

passed #474291
integration testsuites fedora

00:31:21

passed #474295
valgrind bad

00:07:10

passed #474292
valgrind base

00:08:06

passed #474296
valgrind ges

00:06:53

passed #474293
valgrind good

00:10:56

passed #474294
valgrind ugly

00:02:46

 
  Integrate
passed #474297
android universal examples

00:08:12

passed #474298
documentation

00:03:59