Support drag-and-drop in macOS

[yamadapc]

I've made the patches here:

• yamadapc@377fa94

And:

• yamadapc/iced_baseview@a8c41ed

[yamadapc]

This is to support the iced_native::window::Event::FileHovered and iced_native::window::Event::FileDropped events, which can be useful to allow users to drop audio/midi files onto an audio app/plugin.

I've not tested inside of a VST (but it should be fine).