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