Spell fix: visable -> visible

[plajjan]

"visible" is spelled incorrectly in a number of places in the source. Since it's also part of the JSON data being loaded we might want to consider accepting both 'visable' and 'visible', at least over some time, for background compatibility.

[otm]

That is probably a good idea. The cleanest way forward is change 'visable' to 'visible' everywhere in the code and add decorator to the Graph.load function that changes 'visable' to 'visible'.

I can probably do the code change fairly quickly, but testing will take a bit of time as they are lacking, to say at least.