osx: add support for mousewheel, non-primary buttons and gestures

[sqweek]

I got to spend some quality time with an actual OSX box yesterday, and added support for some mouse/gesture events which were previously ignored:

• mouse clicks/drags with the non-primary button
• mouse wheel events
• standard touch gestures (pinch/rotate/scroll)

This PR adds support for all three; I consider gestures essential for providing a quality ux on the platform. Notes:

• cocoa calls [NSResponder scrollWheel ]for both mouse scroll wheel events and scrolling gestures (two-finger swipes). I attempt to distinguish between the two via [NSEvent hasPreciseScrollingDeltas] which worked in my limited test scenarios.
• osx seems to have a reversed scroll-wheel convention compared to other platforms. ie. scrolling the mouse wheel down takes you upwards, toward the top of the document. I'm guessing this is to be consistent with scroll gestures (scrolling down with two fingers does intuitively shift the document down/viewpoint up). Currently I've plumbed a mouse-wheel-down scroll through as a WheelDownButton, but it may be worth considering reversing the mapping so that wde programs can transparently adapt to the native platform?
• the scrollWheel event also supports left and right scroll wheels, which currently get dropped on the floor. This may be obscure enough not to worry about (left/right scrolling gestures are supported).
• MagnifyEvent would perhaps be better named PinchEvent to avoid imprinting expected UI behaviours, but I think that ship has sailed

[tcolar]

FYI, I tried this (except gestures) and works properly on Lion. PS : Yes OSX scrolling is "backward", that's just how they do it.

[sqweek]

Thanks for confirming! I wasn't entirely sure whether the scroll wheel thing was a glitch with my hardware, as the only usb mouse I could test with is an obscure old trackball model :)

[sqweek]

Decided to flip the mouse wheel events around after all. It feels like a lie but provides a better user experience overall IMO.