Fix keyboard input on macos

[elcritch]

Turns out that NSApplication doesn't send keyUp events when command or other modifiers are being held down. Seems really weird, but it's an old issue.

This means Windy can't remove the non-modifier keys released before modifiers are released.

Here's some background info:

• https://stackoverflow.com/questions/24099063/how-do-i-detect-keyup-in-my-nsview-with-the-command-key-held
• https://lists.apple.com/archives/cocoa-dev/2003/Oct/msg00442.html
• https://github.com/andlabs/ui/blob/bc848f5c4078b999dbe6ef1cd90e16290a0d1c3a/delegateuitask_darwin.m#L46

Initially I tried to follow their approaches of sub-classing NSApplication. However, it seems Window uses a global NSApp. Instead, since Windy already handles all events in pollEvents that was much simpler to implement.