Refactor custom scripts to not be stored in one giant vector of lines

This commit refactors custom level scripts to no longer be stored in one
giant vector containing not only every single script name, but every
single script's contents as well. More specifically,
scriptclass::customscript has been converted to an std::vector<Script>
scriptclass::customscripts (note the extra S), and a Script is just a
struct with an std::string name and std::vector<std::string> contents.

This is an improvement in both performance and maintainability. The game
no longer has to look through script contents in case they're actually
script names, and then manually extract the script contents from there.
Instead, all it has to do is look for script names only. And the
contents are provided for free. This results in a performance gain.

Also, the old system resulted in lots of boilerplate everywhere anytime
scripts had to be handled or parsed. Now, the boilerplate is only done
when saving or loading a custom level. This makes code quality much,
much better.

To be sure I didn't actually change anything, I tested by first saving
Dimension Open in current 2.3 (because current 2.3 gets rid of the
awful edentity whitespace), and then resaved it on this patch. There is
absolutely no difference between the current-2.3-resave and
this-patch-resave.

desktop_version/src/Script.h

@@ -9,6 +9,12 @@
 #define filllines(lines) commands.insert(commands.end(), lines, lines + sizeof(lines)/sizeof(lines[0]))
 
 
+struct Script
+{
+    std::string name;
+    std::vector<std::string> contents;
+};
+
 class scriptclass
 {
 public:
@@ -63,7 +69,7 @@ public:
     int i, j, k;
 
     //Custom level stuff
-     std::vector <std::string>  customscript;
+    std::vector<Script> customscripts;
 };
 
 extern scriptclass script;