gtkgstwidget: add ready-to-show signal.

Add a way to know when the widget is ready to show video content.

Closes issue #528
6 jobs for 528 in 28 minutes and 34 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
passed #63045
gst indent

00:00:27

passed #63044
manifest

00:00:19

 
  Build
passed #63046
build fedora x86_64

00:03:11

 
  Test
passed #63047
check fedora

00:15:32

 
  Full Builds
passed #63048
build android arm64 api28

00:02:24

passed #63049
build cerbero fedora x86_64

00:09:22