Reimplement macos file picker using native APIs in the app rather than calling out to osascript

[Hixie]

The current approach results in this weird UI experience where the file picker isn't focused correctly, and where there's a new system process for the dialog that is separate from the actual app. It also runs into problems if osascript is disabled for some reason (e.g. security policy).

There doesn't seem to be any reason why we could implement this using regular message channels to code implemented natively in the app, like a normal plugin.

[navaronbracke]

@Hixie I'll be able to tackle this myself, after fixing up a large part of the platform interface.

[stuartmorgan]

Probably one of #1145 and this should be closed, since they seem to be describing the same thing.

[navaronbracke]

I'll keep this one open, since the title signifies better what is meant to be done in my opinion.