Gecko keeps designMode on during HTML insertion

Just to bring you back into the loop, WymClassTrident7 inherits most of its methods from WymClassGecko. This means that when I removed WymClassGecko's `rawHtml` method, it was removed for WymClassTrident7, as well.

So, adding it here is not new behavior for WymClassTrident7, but retaining its current behavior.