First version of new visual, CSS3-based drop shadow property.
Integrating pull request feedbacks from bschmid

Cherry-pick of commit 293a14d8bc9eb8e461e8eb13f46045d6363ecc78
Fixing minor issues

cherry pick of commit b980eef65
Create BoxShadowTest.java

GUI test for feature BoxShadow
Fixing some minor details...
Added API documentation for BoxShadow property
Merge branch 'feature/css3-boxshadow' of https://github.com/chrismay2/echo3

Reviewed version of a CSS3 based visual property for drop shadows on
buttons and window based on XBGSofts implementation.
For discussions refer to https://github.com/echo3/echo3/pull/24