Document main_menu state machine, name window constants

The new names for the constants (and newly named functions) give a much
better understanding of what the window system is trying to do.

src/contest_painting.c

@@ -279,7 +279,7 @@ static void InitContestPaintingWindow(void)
     SetBgTilemapBuffer(1, AllocZeroed(BG_SCREEN_SIZE));
     gContestPaintingWindowId = AddWindow(&gUnknown_085B07EC);
     DeactivateAllTextPrinters();
-    FillWindowPixelBuffer(gContestPaintingWindowId, 0);
+    FillWindowPixelBuffer(gContestPaintingWindowId, PIXEL_BUFFER_TRANSPARENT);
     PutWindowTilemap(gContestPaintingWindowId);
     CopyWindowToVram(gContestPaintingWindowId, 3);
     ShowBg(1);