UI: Add ability to copy-paste scene item transforms

This commit adds the ability to copy a scene item's transform and crop
settings and paste those settings onto another scene item in any scene
or scene collection. It also changes the menu shortcut key for the
Transform action "Center to screen" from "C" to "n" because "C" is the
standard shortcut key for "Copy" in most other applications.

Closes jp9000/obs-studio#719
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f