UI: Disable Copy Filters in Audio Mixer for source with no filters

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When you right-click on a source in the Sources list that does not have
any filters, the "Copy Filters" item is disabled. When you right-click
on an audio source in the Audio Mixer that does not have any filters,
the "Copy Filters" item is enabled. This fixes the Audio Mixer context
menu to behave like the Sources context menu.

Fixes #4790.

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