Animation: Autopin

(These two commits are in one MR because one builds on top of the other.)

• "Show in timeline" language has been changed to "Pin to timeline" across both the code and the user interface. Adding a layer to the timeline should now be referred to as "pinning" and removing it should be referred to as "unpinning". This should hopefully make things more clear for users. BUG:407416

• New option: "automatically pin new layers to timeline". Users can now decided whether they want new layers to be pinned by default or not. Added a user setting for Auto-Pinning paint layers to the animation docker to general preferences > miscellaneous. BUG:414690 BUG:413805

Test

Test creation of layers with the automatic pinning option on and off.

Formalities Checklist

• I confirmed this builds.
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