PIVOT-935: Allow changes to styles in the predefined Alert class without
having to change the global theme colors.

This change exposes the internal Border component to TerraAlertSkin.java
and then adds accessor methods for that Border's background and foreground
colors (which are accessed via the normal "style" mechanisms).


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1570316 13f79535-47bb-0310-9956-ffa450edef68