More work on child windows

- Popovers are implemented as child windows.
- Renamed _CPAttachedWindow/_CPAttachedWindowView to _CPPopoverWindow/_CPPopoverWindowView, since that is its only use.
- The default for CPView -acceptsFirstMouse is now NO, per Cocoa. Subclasses override this as necessary.
- _CPWindowView -hitTest returns self it the mouse is within a resize region, which may be outside the window's frame.
- Fixed an off by one bug in CPDomWindowLayer -insertWindow:atIndex:, where inserting a visible window behind a window it is already behind would cause it to move up one from its intended position.
- Updated the ChildWindows and CPPopover test apps.

AppKit/CPMenu/_CPMenuBarWindow.j

@@ -320,6 +320,11 @@
     [self tile];
 }
 
+- (BOOL)acceptsFirstMouse:(CPEvent)anEvent
+{
+    return YES;
+}
+
 - (void)mouseDown:(CPEvent)anEvent
 {
     var constraintRect = CGRectInset([[self platformWindow] visibleFrame], 5.0, 0.0);