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Don't fullscreen those who set it to false.

This makes things much simpler and extensible. We also avoid playing
with the stage color, rather just replace the _background with
a solid if we're in fullscreen.

The gnome-shell tweener wrapper has some niceties, including frame
dropping on slow systems and automatic tweener destroying if the actor
goes away.

Also, it allows global slowdown factor support, which i've added if
the SUSHI_SLOWDOWN_FACTOR env var is set.

This lets us easily control things like overall alpha

This is needed, because the basename might not be proper utf8

The load order of the viewers is essentially random, so we can't
rely on it being imported in only one viewer, as they all use it.

All the viewers share a context, so if any two viewers import the
same module, then you'll get an error about redeclaration if const
is used.

It's much faster this way.