add upgrade rules for legacy animations and setter for z

[riknoll]

fixes #6616

this adds two upgrade rules for very old projects and fixes an exception in another. the new rules are explained in the comments

this has actually been broken for a long time, but new blockly has surfaced the errors. old blockly used to be looser with field validation so it wouldn't overwrite invalid field values when the workspace loads, even though it wouldn't render correctly. for example, if you open the project in the linked issue in live you'll see that all of the blocks setting the Sprite.z property actually say x, but they still compile to z because the invalid value isn't overwritten.

new blockly doesn't allow invalid field values in dropdowns, and instead defaults to choosing the first available item in the list of options and overwriting the invalid value with it. the project ends up looking the same in beta but the properties compile to the displayed value x instead of z. same with the animation dropdowns

[srietkerk]

LGTM