fix (CPWindow): prevent empty content view when resizing a window

Previously, windows with no minimum size could be resized
by the user such that the content view was completely hidden, and then
resized such that the window was effectively turned inside out.
This obviously needed to be fixed.

Now, when the user resizes a window, each _CPWindowView subclass
is asked for the minimum resize size, with the size being additively
set by each subclass. This minimum size takes into account things
like the title bar, divider line, and close button, and always leaves
at least 2px height for the content view.

The user-requested size is then pinned to the minimum resize size.

In the process of fixing this bug, I discovered in the Cocoa docs
that the CPWindow -setFrame: methods should ignore minSize and maxSize.

Other changes:
- Take the divider height into account in contentRectForFrameRect for titled windows.
- Use _CG macros and fix formatting.

Closes #1753