Refactor scriptclass::startgamemode

This overhauls scriptclass::gamemode massively.

The first change is that it now uses an enum, and enforces using that
enum via using its type instead of an int. This is because whenever
you're reading any calls to startgamemode, you have no idea what magic
number actually corresponds to what unless you read startgamemode
itself. And when you do read it, not every case is commented adequately,
so you'd have to do more work to figure out what each case is. With the
enum, it's obvious and self-evident, and that also removes the need for
all the comments in the function too. Some math is still done on mode
variables (to simplify time trial code), but it's okay, we can just cast
between int and the enum as needed.

The second is that common code is now de-duplicated. There was a lot of
code that every case does, such as calling hardreset, setting Flip Mode,
resetting the player, calling gotoroom and so on.

Now some code may be duplicated between cases, so I've tried to group up
similar cases where possible (most notable example is grouping up the
main game and No Death Mode cases together). But some code still might
be duplicated in the end. Which is okay - I could've tried to
de-duplicate it further but that just results in logic relevant to a
specific case that's located far from the actual case itself. It's much
better to leave things like setting fademode or loading scripts in the
case itself.

This also fixes a bug since 2.3 where playing No Death Mode (and never
opening and closing the options menu) and beating it would also give you
the Flip Mode trophy, since turning on the flag to invalidate Flip Mode
in startgamemode only happened for the main game cases and in previous
versions the game relied upon this flag being set when using a
teleporter for some reason (which I removed in 2.3). Now instead of
specifying it per case, I just do a !map.custommode check instead so it
covers every single case at once.

desktop_version/src/Editor.cpp

@@ -2796,7 +2796,7 @@ void editorinput(void)
                         graphics.backgrounddrawn=false;
                         ed.returneditoralpha = 1000; // Let's start it higher than 255 since it gets clamped
                         ed.oldreturneditoralpha = 1000;
-                        script.startgamemode(21);
+                        script.startgamemode(Start_EDITORPLAYTESTING);
                     }
                 }
             }