don't ineherit from Static

[willmcgugan]

• Don't inherit Pretty from Static
• Renamed Static.renderable to Static.content

[TomJGooding]

The Static widget has changed a lot recently so maybe I just haven't kept up, but I don't understand why there's an update method and now a content setter? Is this just for backward compatibility?

You can also get weird results like this:

static = Static("Old content")
static.content = "New content"
assert static.content == "New content"
# AssertionError: assert 'Old content' == 'New content'