Support drag-and-drop in macOS

[yamadapc]

I've made the patches here:

• https://github.com/yamadapc/baseview/commit/377fa94bce605caaf8475952ad6d167bd8761b10

And:

• https://github.com/yamadapc/iced_baseview/commit/a8c41ed26727208f45a9263f7396043652f7d779

[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).