New: CPAlert supports custom didEnd selector or block.

Previously, unless you were using a sheet alert, there was no way to set a custom dismissal handler for an alert. In Cocoa alerts are synchronous, in Cappuccino they are asynchronous, which meant that a single dismissal handler had to handle all possible non-sheet alerts.

This commit adds several methods which add two ways to specify a custom dismissal handler: via delegate/selector, and via a block (function). The new methods are:

setDidEndSelector:
runModalWithDidEndBlock:
beginSheetModalForWindow:didEndBlock:

Note that didEnd blocks are transient and take precedence over didEnd selectors.

The CPAlertTest demo was updated to fix some bugs and added the ability to run the tests using blocks.