Move screen::Screen into Umbra as UmbraScreen

[simondevenish]

A useful generic helper class. The only treeburner specific code relates to method TCODImage* loadChapterPicture(bool big = false); and is only used by screen/end.cpp where it can be moved into

[HexDecimal]

I see what you mean, but there are still some issues I'd like to clear up. UmbraScreen would be redundant. Anything in the umbra:: namespace won't need an Umbra prefix. Also:

• Screen needs to be changed to use SDL for events.
• Permissions with these classes should be cleared up. There's a lot of weird stuff going on. I'd like to consult the C++ Core Guidelines on how to handle class permissions here.

[HexDecimal]

Looking more closely at Screen the less I like the idea of porting it to Umbra. The internals are messy and the protected variables have leaked everywhere.

Pictures are hard-coded into Screen and there's a complicated fading system which has a lot of parts exposed. The fade system is probably fixable by making a public interface to the internal variables and then making those private.