add support for shift, option (alt), control and command (meta) modifier keys

[jbg]

Passes through the NSEventModifierFlag values for shift, option, control and command as Android metaState flags.

[stuartmorgan-g]

Just one nit, otherwise looks good.

[krisgiesing]

Why are we using Android-defined constants here? Shouldn't we use a platform-independent description of the modifier key state?

[stuartmorgan-g]

Shouldn't we use a platform-independent description of the modifier key state?

There isn't one, so currently desktop on all platform is sending Android. See line 382 in this file, for instance.

[krisgiesing]

I see, thanks for the clarification. I think we'll want to make one ourselves then, at some point.

[stuartmorgan-g]

Yes, the current system of using system channels and internal implementation details should be replaced with a real embedding API (essentially #47, but for key events rather than text input).