Change corridor groups to wall/floor/ceiling instead of flat/upward/downward

[vrad-exe]

Currently, corridors are grouped into the orientation categories "flat", "upward", and "downward". Flat always refers to the corridors used on walls, but upward and downward refer to the direction the player moves through the corridor, which means they have different meanings for entrances and exits: for entrances, upward refers to floor corridors and downward refers to ceiling corridors, but for exits it is reversed.

While I get the idea here, I think this system is more harmful then helpful, and it should just be changed to group corridors based on the surface they attach to.

• Users are probably going to think about corridors based on the orientation the door is placed in in their map, so the current grouping adds an extra layer of confusion where they have to think "wait, do I want upward or downward corridors"?
• For new users who aren't familiar with now the corridor system works, it's not immediately clear that "upward" and "downward" refer to floor/ceiling corridors in the first place. Vertical corridors appear nowhere in the official campaigns or most community maps, so most players are probably going to assume these refer to corridors with stairs in them or something. With wall/floor/ceiling, it's immediately clear these refer to surfaces the door is attached to.
• In terms of UI, it creates an inconsistency with how the various "tabs" in the corridor selector window behave. With the first two sets of tabs, they always correspond to a single variable in the user's maps: the game mode, and the door type respectively. But with the third set of tabs, the upward and downward options correspond to both the attachment surface and the door type. If the user has downward entry (ceiling entry) selected, then switches to exit, now they are looking at floor exits.

[vrad-exe]

Internally the current group names would probably be kept for backwards compat, but maybe the new names could be added as aliases or something