[FIX] RenderManager: Make hidden placeholders identifyable in CSS

Invisible placeholders, rendered when a control has its visible
property set to false, now have the class "sapUiHiddenPlaceholder"
in addition to the "display: none" style attribute.

Change-Id: If59549bcef233536202f1e39225c4ef5bc04e071
Merge "[FIX] RenderManager: Make hidden placeholders identifyable in CSS"