Fix #357: Fix resizeWithOldSuperviewSize to work more like Cocoa

Without this fix, CPView's resizeWithOldSuperviewSize: would simply resize subviews at the rate of 1 pixel for every 2 pixel change.

This change introduces a new method of resizing views to a ratio of the size of the superview.

Patch contributed by @davidkhess
Merge pull request #1821 from ahankinson/fix-issue357

Fix #357: Fix resizeWithOldSuperviewSize to work more like Cocoa