Formatting
Fixed: if a sheet's parent window frame was changed, the sheet did not reposition

Previously, sheets were only repositioned if the parent window's setFrameOrigin: method was called. This did not cover all possible cases for parent window repositioning.

Now the sheet is repositioned any time the parent window's origin changes.
Fixed: sheet top shadow would not adjust when parent window was resized

It is possible to resize the parent window of a sheet while it is open. Previously, the sheet's top shadow was not adjusted when the parent window was resized, which could lead to it being out of synch.

Now the sheet shadow is adjusted whenever the parent window's size changes.
Fixed: resize masks for a few views in AttachedSheet2 demo were not set

Previously they floated, now they are anchored.
Fixed: sheet opening animation was not cleaned up when sheet immediately closed

Previously, if an event occurred during the sheet opening animation that would close the sheet, the animation was not properly cleaned up and the subsequent closing animation would die horribly.

The animation is always cleaned up with this commit.