rtspsrc: handle seek event on the element

Without this, the user has to wait for rtspsrc to have sent a PLAY
request and exposed its pads before seeking it.
19 jobs for rtspsrc-seek-handler in 37 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #388791
gst indent

00:00:33

passed #388790
manifest

00:00:44

 
  Build
passed #388797
build cerbero android universal

00:11:34

passed #388798
build cerbero cross win32

00:08:16

passed #388799
build cerbero cross win64

00:06:17

passed #388796
build cerbero fedora x86_64

00:07:35

manual #388801
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #388800
gst-macos-10.14
build cerbero macos x86_64

00:06:47

passed #388792
build fedora x86_64

00:06:14

passed #388793
build nodebug fedora x86_64

00:06:08

passed #388794
build static fedora x86_64

00:13:59

passed #388795
build static nodebug fedora x86_64

00:12:30

 
  Test
passed #388802
check fedora

00:03:37

passed #388803
integration testsuites fedora

00:15:52

passed #388805
valgrind bad

00:11:08

passed #388806
valgrind ges

00:08:05

passed #388804
valgrind good

00:12:26

 
  Integrate
passed #388807
android universal examples

00:07:12

passed #388808
documentation

00:04:08