Add conditional compilation symbol NO_UNITTESTS so that Eto.Test can be compiled without a reference to NUnit. Full compilation occurs by default when the symbol is not defined.
DrawableHandler: a long-overdue cleanup of some code I had previously added to support text handling on Drawable which is iOs-specific and turns out to be not easily generalizable. Therefore this code has been removed to an app handler.
Make DrawableHandler.Create virtual so that a derived type can override it.