Adds a new "setSheetOffset" API to the `dialog` module, which allows you to change the attachment point for sheets on Mac OS X. I put the API on the dialog module, even though Mac OS X requires that the native window hold and return the desired offset.
1. I was originally hoping to make this an argument on the actual dialog.show* calls, but it seems the parameter set is defined in `libchromiumcontent` and I wasn't sure it would be appropriate to add there?
2. The API could also be on the BrowserWindow (eg `BrowserWindow.setSheetOffset`). I don't have a strong preference, but I think it's more discoverable on the `dialog` module.
In N1, we want to implement the famous "swipe to archive" action on threads in the user's inbox. Chrome exposes `scroll` and `wheel` events, but these aren't sufficient to implement the interaction because the element needs to "snap" when the user lifts their fingers from the trackpad, not when they / we stop receiving `wheel` / `scroll` events. These events may stop before the user lifts their fingers, or continue after the user has lifted their fingers if they had enough momentum for the gesture to continue.
This exposes BrowserWindow `scroll-touch-down` and `scroll-touch-up`, which fire immeditaely when the user touches two fingers to the trackpad, and again when the user lifts their fingers. Combined with the existing wheel event should allow for "swipe-to-archive" and other similar interactions.
Note: This is only implemented on Mac OS X and the events don't fire unless you're using a trackpad!
Related: #1486, #2683, https://github.com/nylas/N1/issues/541
Previously we implemented draggable region by tracking mouse position,
it is buggy and causing some problems. But it is interesting that until
this didn't cause troubles until recently.
