Go over 'all' (as far as I can tell) animations in Textual. Most of them should only run when the env variable TEXTUAL_ANIMATIONS is set to FULL.
A few animations may run on the level BASIC, which are animations that don't delay content appearing:
 - indeterminate progress bars
 - loading indicators
 - button presses
 - tab underlines
 - switch toggles
 - all (?) types of scrolling.

These animations are completely disabled when the env var is NONE.

The indeterminate progress bar displays a full, static bar and the loading indicator displays a string 'Loading...'.

Many animation-related methods also grew a keyword parameter 'animate_on_level' that establishes the minimum level for said animation to take place.

By using 'constants.SHOW_ANIMATIONS' instead of importing the constant directly we make it easier to patch for testing.

See: https://mathspp.com/blog/til/patching-module-globals-with-pytest

The original issue (#3992) asked for a property on 'App' that allows controlling whether that app's animations should be played or not.

Check that animations that should happen on the BASIC level do happen at that level and don't happen on the NONE level.

See review comment: #4062 (comment).

See #4062 (comment).