[TECHNICAL-SUPPORT] LPS-37302 Display Page can not be selected in Web Content dialog if staging with page versioning is enabled

Hi Zsolt,

I'm sending this pr to you, as you are already familiar with this issue.
I'm copying here Vilmos' thoughts: 

"I have to revert @brianchandotcom's source format because QA reported that this issue still reproducible, but my original fix solves it. I checked the cause and realized that the changed method name in the SF commit caused the regression, as we have to have the same method in LayoutRevisionImpl as we have in LayoutImpl, namely isContentDisplayPage, because the LayoutStagingHandler processes the method calls on the LayoutRevision object if only it implements the called method. On client side we usually call layout.isContentDisplayPage() but layout could be easily a proxy object and LayoutStagingHandler is responsible to call the method on the proper object.

Please take these in account if you want to modify the method name."

Thanks,
Tamás