Fixed: missing theme image, window sizing/moving was broken.

After bbfaac5, all kinds of strange behavior occurred with window sizing and moving due to overzealous constraining.

Cocoa does not constrain the frame of hidden windows. When a hidden window is ordered in, it is constrained to the usable screen content rect. A visible window has its height constrained when its frame is set. Its origin is constrained such that a minimum margin at the left, right and top is visible, and the top is constrained to be below the menu bar.

This commit fixes a number of problems related to window moving and resizing that were introduced by bbfaac5:

- Sheets are not constrained at all.
- Moving a window does not constrain its size.

In addition, when a window is moved, tracking is relative to the initial click point, which provides proper behavior when the movement is constrained and the mouse keeps moving.