make sure MainWindow is not shown before Renderer is ready

MainWindow is set to be shown centered on screen, but if we show it before the contained Renderer is ready (i.e. when it has gotten its final width and height) then that new empty window will be centered for a moment but just after that, the window gets resized to make room for the contained Renderer final dimensions.

This window enlargement ruins the centering, because the window x,y origin is still the same.

The Evince Renderer was not suffering this bug because it subclasses GtkScrolledWindow which probably has a different handling for that. But the Image Renderer which just subclasses GtkDrawingArea was suffering this bug.

The fix is just to make sure we don't show the MainWindow until the Renderer has signaled it's ready (i.e. it has its definitive width and height).

Although we want to have MainWindow not visible initially, we still had to call show_all() on it briefly so that Renderers which use Sushi.MediaBin (like gst.js and audio.js) don't show an extra window, that was probably the original motivation for showing the window before ready signal.

Closes #93 (closed)