Enumify all fade modes

This removes the magic numbers previously used for controlling the fade
mode, which are really not readable at all unless you already know what
they mean.

0: FADE_NONE
1: FADE_FULLY_BLACK
2: FADE_START_FADEOUT
3: FADE_FADING_OUT
4: FADE_START_FADEIN
5: FADE_FADING_IN

There is also the macro FADEMODE_IS_FADING, which indicates when the
intention is to only check if the game is fading right now, which wasn't
clearly conveyed previously.

I also took the opportunity to clean up the style of any lines I
touched. This included rewriting if-else chains into case-switches,
turning one-liner if-then statements into proper blocks, fixing up
comments, and even commenting the `fademode == FADE_NONE` on the tower
spike checks (which, it was previously undocumented why that check was
there, but I think I know why it's there).

As for type safety, we already get some by transforming the variable
types into the enum. Assignment is prohibited without a cast. But,
apparently, comparison is perfectly legal and won't even give so much as
a warning. To work around this and make absolutely sure I made all
existing comparisons now use the enum, I temporarily changed it to be an
`enum class`, which is a C++11 feature that makes it so all comparisons
are illegal. Unfortunately, it scopes them in a namespace with the same
name as a class, so I had to temporarily define macros to make sure my
existing code worked. I also had to temporarily up the standard in
CMakeLists.txt to get it to compile. But after all that was done, I
found the rest of the places where a comparison to an integer was used,
and fixed them.

desktop_version/src/main.cpp

@@ -656,7 +656,7 @@ int main(int argc, char *argv[])
             script.startgamemode(22);
         }
 
-        graphics.fademode = 0;
+        graphics.fademode = FADE_NONE;
     }
 #endif