UI: Enable "Paste Filters" after copying from scenes or mixer

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Fixes an issue where if no filters had previously been copied and the
user copies a filter from either the scene tree or the audio mixer, the
"Paste Filters" action would not be available until the Edit menu
reloads (e.g., by selecting another source in the source tree).

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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f