Fixed: sheets should not be resizable from the top

Previously, sheets could be resized from the top, which would destroy the illusion of being attached to the top of the parent window.

With this commit, sheets are prevented from being resized by the user from the top edge.
Fixed: attached sheet shadow had extra width, was too dark

Previously the attached sheet shadow (that appears below the title bar of the window a sheet is attached to) was 9x8, and it was quite dark.

With this commit the width has been reduced to 1 (to reduce the image size) and it has been lightened up.

Also changed "height-shadow" theme attribute to "shadow-height".
Fixed: sheet animation was resizing the sheet to zero, causing problems

Previously sheet animation was performed by sizing the sheet from zero to full height when animating in, and then back to zero height when animating out. This caused problems with the resize algorithms, which are not designed to deal with sizing from zero.

Analyzing Cocoa behavior revealed that sheets slide in and out with no resizing.

With this commit, a general purpose mechanism for clipping a window to a rect was added to CPDOMWindowLayer. Clipping was put in CPDOMWindowLayer because that is the class that deals with windows at the global level. CPWindow uses the clipping mechanism to slide the sheet in and out, thus avoiding all resizing problems.

Fixes #1840
Fixed: sheet resizing displayed incorrectly

Previously, when a sheet was resized, there were a number of problems:

- Sheets in Cocoa resize their width symmetrically, such that they always remain centered. This was not happening in Cappuccino.

- When a sheet is resized wider than its parent window, the shadow at the top of the sheet was growing wider than the bounds of the parent window's title bar, which makes no sense since the shadow is supposed to be cast by the title bar.

- Sheets had no top border, so when they were resized wider than the parent window, the top edge looked like it was cut off. In Cocoa, sheets have a top border that sits under the title bar of the parent window, and which is visible when the sheet is resized wider than the parent window.

This commit addresses these problems as follows:

- Sheet width is now resized symmetrically.

- Sheet resizing is pinned to the screen bounds.

- The top sheet shadow is pinned to the width of the parent window's content view.

- Sheets now have a top border.

In addition to these changes, a redundant sheet top shadow was eliminated from one of the _CPWindowView subclasses.

Fixes #1846