Fixed: windows were allowed to extend beyond the usable screen content.

Previously, no bounds were placed on the placement or sizing of windows, which allowed them to extend beyond the usable screen content (the area bounded by the menu bar at the top and the platform window on the other sides).

Cocoa enforces the following restrictions:

- When a window is ordered in, it is forced within the usable screen content (below the menu bar) and its size and width are clipped to the usable screen content as well.

- If a window is already visible, any methods that move or resize the window will ensure the top and bottom of the window are within the usable screen content. The width is unchanged.

This commit implements Cocoa's restrictions in CPWindow.

Fixes #1690