Load an unlimited amount of sounds

This commit allows the usage of new "extra" sounds in levels for use in
custom level scripting. Including `squid.ogg` inside of your sounds
folder will now allow for the usage of the command `playef(squid)`.
Additionally, you can play built-in sounds by name this way too,
instead of having to memorize the id (`playef(rescue)`).

However, you CANNOT play extra sounds through using their numerical
IDs, since those can shift around depending on the filesystem (or if
someone adds a new sound). Playing an extra sound by ID will NOT play
the sound.

This is another attempt at #902, being loosely based off of that PR.

Co-authored-by: Fussmatte <doormattion@gmail.com>

desktop_version/src/Music.h

@@ -95,6 +95,10 @@ public:
     int haltedsong;
 
     void playef(int t);
+    void playefid(const char* id);
+    bool soundidexists(const char* id);
+    bool soundisextra(int t);
+    const char* getsoundid(int t);
     void pauseef(void);
     void resumeef(void);