[D11] frontend changes wip fix #3; PagedUI should not have been a singleton, FrontendUI on the other hand should be. This opens up the possibility of creating more PagedUIs (which needs to be done anywhere FrontendUI is currently being accessed in game). Also remove confusing casts that are no longer necessary. Remove references to global UIs when the parent UI can be known locally, makes it much easier to reuse UIs in several places.