Remove text labels from media buttons.
Update playPauseButton to use new _create_media_button()

No new functionality, only code cleanup. (DRY)
Rework quit semantics.

Quitting the application now requires all players being stopped to
prevent unintended termination.

If playing, tell the user to stop the players, if stopped, ask for
confirmation and really quit.

The window destroy event still conveniently terminates the application,
but the window close button won't be visible in production mode (-f).
Remove quit button

Quit is now handled via Control-q.
Remove obsolete quit_cb().

Without a quit button, we need no quit callback().
Allow compilation with gstreamer-0.10

Use make OLDGSTREAMER=1 to build against gstreamer-0.10
Update README and mention gstreamer-0.10