Add key commands for keyboard navigation

[1ec5]

MGLMapView should declare its ability to become the first responder and register some key commands to aid with keyboard navigation on the iPad.

For consistency with the macOS SDK, some basic commands come to mind:

• To scroll, press the arrow keys.
• To zoom in, press ⌘+ or ⌥↑.
• To zoom out, press ⌘− or ⌥↓.
• To rotate, press ⌥← and ⌥→.
• To snap to north, press ⌘↑.

There doesn’t appear to be a way to implement gesture recognizers that depend on modifier keys, like shift-panning in the macOS SDK.

/cc @friedbunny

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[1ec5]

Keyboard support will potentially become more important as UIKit comes to the Mac.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.