Env var to modify animation

[willmcgugan]

Add an environment variable to set animation behaviour.

TEXTUAL_ANIMATION should be one of the following values:

NONE for no animation at all.
BASIC for scrolling animation and other animation that doesn't delay content appearing (such as the tab underline).
FULL for all animations.

Any other values should be equivalent to FULL

This should also be replicated in App with a property, so it could be set programatically.

[rodrigogiraoserrao]

This looks like it'll take a couple of days because I need to figure out a way for the animations code to determine whether or not an animation is “basic”.
(If it were just NONE vs FULL, it'd be simpler.)

[rodrigogiraoserrao]

Looks like we don't want TEXTUAL_ANIMATIONS to interfere with the parameter delay that is set on some animations.
A delayed animation with TEXTUAL_ANIMATIONS set to NONE will be delayed for the correct amount of time and then it happens instantaneously.

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