fix #1354: Overlay on section not properly disposed when refresing parent section
fix #1349 aria.utils.dragdrop.Drag on Chrome with elements in position:fixed

This commit fixes a problem inside the getOffset method of the aria.utils.Dom class. We use the offsetParent property to compute the element position, but for a fixed element in Chrome the offsetParent is null and it breaks the code.
fix #1360 ButtonVerticalAlignTestSuite

Allowing tests from ButtonVerticalAlignTestSuite to pass for other skins
than atskin and atflatskin.
fix #1361 Fixing Robot tests failures on Firefox 3.x

Robot tests are supposed to be ignored on Firefox 3.x, however, there
were still indirect errors after commit e55822502a93ba95fdcf3b07f885d543c99692c0.