[Map Editor] Add 3d floor/polyobject rendering in the 3d view

[darkhog]

SLADE Version: (3.2.1) Operating System: Windows

Issue Details: Doom builder already has feature to render 3d floors directly in the 3d view, making editing them easier (not sure about polyobjects, but hey, Slade can one-up DB in that regard!). Code to do that could be either borrowed from the Doom Builder or just from GZDoom proper.