Create Wait Animation

[eulertour]

Creates a Wait Animation which has the same effect as Scene.wait(). Also removes some of
the methods within Scene and the caching logic which is specific to waiting.

Depends on #562

[huguesdevimeux]

This seems good to me.
Although there is no apparent reason it shouldn't work, did you try the caching mechanism with Wait with condition ?
Maybe a test for that with hashing is needed.

[behackl]

The changes in the code look good to me. There are a few issues with the docstrings, please consider my suggestions.

[leotrs]

Let's add @behackl's suggestions and fix the conflict and then merge :)

[eulertour]

This depends on #614 since it's currently based on the faulty logic for finding static mobjects.

[eulertour]

@behackl This PR is ready when you are

[behackl]

Ah, I missed 6c79273 -- good to go. 👍

[cobordism]

This PR broke the AddTextLetterByLetter animation (among others)

class lbl(Scene):
    def construct(self):
        t2 = Tex("this is test").shift(DOWN)
        self.play(AddTextLetterByLetter(t2[0]))
        #this works on the releases branch, but not on master