Added, Visual mode: added support for FLATSPRITE, ROLLSPRITE, WALLSPRITE and...

Added, Visual mode: added support for FLATSPRITE, ROLLSPRITE, WALLSPRITE and DONTFLIP sprite rendering flags. Removed the older GLOOME-specific implementation.
Changed, "Align Things to Nearest Linedef" action: the action can now also align things with FLATSPRITE and WALLSPRITE flags (previously only things with attached models were aligned).
Changed, "Align Things to Nearest Linedef" action: the action can now align things to the middle part of double-sided linedef, if it has a middle texture assigned.