Add mouse button support to the macOS shell

[stuartmorgan-g]

Uses the new embedding API support for device type and buttons to pass
appropriate mouse button events, allowing for right click, middle click,
etc.

Also fixes some edge cases where macOS event delivery violated Flutter
requirements by tracking more data about the mouse event stream and
adjusting the sent events as necessary.

[dkwingsmt]

LGTM except for comments

[dkwingsmt]

Let's see if my understanding is correct:

• If a mouse is pressed in the tracking area, then dragged out (and keeps moving), Cocoa will keep sending drag events with out-of-area positions.
• If the mouse is then dragged back in without releasing buttons, it will send a mouseEntered while keeping sending drag events.

If the statements above are correct,

• Should we stop sending drag events with out-of-area positions to Flutter, since they should be considered illegal data? Also perhaps to accommodate shells that can not detect out-of-area moves.
• Should we set _mouseState.has_pending_exit to false in mouseEntered:, so that the "drag back in" action is considered a normal drag and does not trigger Reset?

[stuartmorgan-g]

If a mouse is pressed in the tracking area, then dragged out (and keeps moving), Cocoa will keep sending drag events with out-of-area positions.

Yes.

If the mouse is then dragged back in without releasing buttons, it will send a mouseEntered while keeping sending drag events.

No, there's no mouseEntered; it's weirdly asymmetrical. (In that scenario, you don't get the mouseEntered until the next time you move the mouse after releasing the button.)

Should we stop sending drag events with out-of-area positions to Flutter

Let's revisit this later, since the patch isn't changing that behavior and we'll need to carefully think about what exactly the right behavior is in the context of yet-to-be-designed functionality like drag-and-drop from Flutter to other apps. (It's not as simple as just not sending events, because we don't want to drop the pointer if Flutter knows a drag is happening in that scenario, but we also don't want, say, an in-window drop target at the edge of the window to stay highlighted.)

Should we set _mouseState.has_pending_exit to false in mouseEntered:

No (per answer 2 above)