Fix errors and improve UX relating to new animation libraries

- Fix a bug causing an error message when a scene containing an AnimationPlayer with a reset track is saved, by correctly referencing the temporary "default" library.
- Make library dropdown in new animation window assign correct library when creating an animation.
- Similarly allow choice of library when duplicating animation.
- Make library dropdown default to library of currently selected animation.
- Make library dropdown show when exactly one library exists, and it isn't [Global]. Include [Global] on the dropdown in this case (will be newly created if dialog is confirmed).
- When appending (x) to avoid New Anim name collisions, correctly check target library instead of [Global].
- Add parentheses when appending x when duplicating animations in the library editor, for consistency.
- Change titles and prompts to be distinct in name/rename/duplicate dialiogs.
- Fix bug in OprionButton.get_selectable_item(true) when last is not selectable.
- Fix issues where animation wasn't found on deletion/rename by correctly prepending library name.
- Remove an extraneous print_line from animation_track_editor.
- Add messages to errors when an animation isn't found.

editor/animation_track_editor.cpp

@@ -2999,8 +2999,6 @@ void AnimationTrackEdit::gui_input(const Ref<InputEvent> &p_event) {
 					}
 				}
 
-				print_line(hovering_key_idx);
-
 				if (hovering_key_idx != previous_hovering_key_idx) {
 					// Required to draw keyframe hover feedback on the correct keyframe.
 					update();