Revert "Merge pull request #2158 from mrbannon/master"

This reverts commit 228a95f776f2e77b1e3f6010e7a819f243762eed, reversing
changes made to cf93b72edd2c1a4236cffc260073910185743d4d.
Merge branch 'master' of https://github.com/cappuccino/cappuccino
FIXED: using setHidden: for a button in buttonbar was not working

Previously, CPButtonBar was not updating its layout after changing the visibility of its buttons.
This patch adds makes CPButtonBar observersing the buttons and relayout if the visibility of one button is updated.
Merge remote-tracking branch 'CappuccinoOrigin/master'