UI: Only defer property updates for input and transition sources

Deferring properties was only ever made for input and transition
sources. As other property dialogs do not have an "Ok" button that would
cause an update to happen, if the deferred flag was set the callback
would never be called.
Also clarifies the docs to reflect this.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f