Replace setblockcolour() argument with const char*

There's no reason it needs to be an std::string here.

Although, realistically, we should be using an enum instead of
string-typing, but, eh, that can be fixed later.

desktop_version/src/Entity.cpp

@@ -1078,7 +1078,7 @@ void entityclass::createblock( int t, int xp, int yp, int w, int h, int trig /*=
 
     if (customactivitycolour != "")
     {
-        block.setblockcolour(customactivitycolour);
+        block.setblockcolour(customactivitycolour.c_str());
         customactivitycolour = "";
     }