Stop menu view controllers from scrolling.
Fix menu popover size on iPad.

Matches existing behavior on smaller devices.
Don't leave popover orphaned when showing original on iPad.
Show Send To in a popover on iPad.

UIActivityViewController should not be presented modally on iPad, per the documentation; it also looks pretty ugly.

Also clean up display of training popover to eliminate shadow fading in background.
Merge pull request #400 from nriley/popovers

iPad popover fixes