Add a "previous page" option to the levels list

This makes it more convenient if you have a large levels directory, as
some people in the VVVVVV custom levels community do.

On the first page, this option will change to be "last page" instead.

Since the addition of another menu option pushes up the list of levels
too close to the selected level data itself, I've had to move the list
of levels down by 4 pixels (but "next page"/"previous page"/"return to
menu" are still in their same position).

This feature was already added to VCE but hasn't been upstreamed until
now.

desktop_version/src/Render.cpp

@@ -46,8 +46,8 @@ void menurender()
       }
       int tmp=game.currentmenuoption+(game.levelpage*8);
       if(tmp>=0 && tmp < (int) ed.ListOfMetaData.size()){ // FIXME: size_t/int! -flibit
-        //Don't show next page or return to menu options here!
-        if(game.menuoptions.size() - game.currentmenuoption<=2){
+        //Don't show next/previous page or return to menu options here!
+        if(game.menuoptions.size() - game.currentmenuoption<=3){
 
         }else{
           graphics.bigprint( -1, 15, ed.ListOfMetaData[tmp].title, tr, tg, tb, true);