Further optimization.
Mark buttons with an action (but not clicked) as Focusable.

This lets Ren'Py transfer focus to them at the start of an interaction,
preventing hovered actions from being called repeatedly.
Add ui.implicit_add for internal use.

This is a function that takes displayables that are being implicitly
added to a widget by ChildOrFixed. We do not want to copy transforms
added this way, which we had been doing, causing problems.