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/Editor.cpp

@@ -1718,7 +1718,7 @@ void editorlogic(void)
         ed.notedelay--;
     }
 
-    if (graphics.fademode == 1)
+    if (graphics.fademode == FADE_FULLY_BLACK)
     {
         //Return to game
         graphics.titlebg.colstate = 10;
@@ -1896,7 +1896,7 @@ static void editormenuactionpress(void)
             //Quit without saving
             music.playef(11);
             music.fadeout();
-            graphics.fademode = 2;
+            graphics.fademode = FADE_START_FADEOUT;
             break;
         case 2:
             //Go back to editor
@@ -1914,7 +1914,7 @@ static void editormenuactionpress(void)
 void editorinput(void)
 {
     extern editorclass ed;
-    if (graphics.fademode == 3 /* fading out */)
+    if (graphics.fademode == FADE_FADING_OUT)
     {
         return;
     }
@@ -2346,7 +2346,7 @@ void editorinput(void)
 
                 if (ed.saveandquit)
                 {
-                    graphics.fademode = 2; // quit editor
+                    graphics.fademode = FADE_START_FADEOUT; /* quit editor */
                 }
                 break;
             }