tools: add simple command-line gst-play utility for testing purposes

tools: add simple command-line gst-play utility for testing purposes

Differs from a plain gst-launch-1.0 playbin uri=... pipeline in that
it can take multiple arguments and as such allows testing of things
like gapless playback, switching between different formats and the
like. Very minimal at this point, we'll probably want to add
interactive controls and more options at some point.

https://bugzilla.gnome.org/show_bug.cgi?id=553520