Lua/Unit: allow modifying of id field for Lua-only units (fixes #2433)

Celticminstrel recommends restricting this to off-map units until we can be sure
changing the ID is safe in all contexts. I don't see why not, though... but this
should be enough for the situation described in the bug (a unit made with copy_unit).

changelog

@@ -35,6 +35,7 @@ Version 1.13.10+dev:
  * Lua API:
    * New wesnoth.unit_types[].advances_to getter.
    * New wesnoth.unit_types[].advances_from getter.
+   * unit.id is now a modifiable field for off-map (Lua-only) units.
  * Multiplayer:
    * Dark Forecast: Fixed broken faction and leader selection.
    * Rename the Khalifate to Dunefolk.

src/scripting/lua_unit.cpp

@@ -431,6 +431,7 @@ static int impl_unit_set(lua_State *L)
 		map_location loc = u.get_location();
 		modify_int_attrib("x", loc.set_wml_x(value); u.set_location(loc));
 		modify_int_attrib("y", loc.set_wml_y(value); u.set_location(loc));
+		modify_string_attrib("id", u.set_id(value));
 	}
 
 	std::string err_msg = "unknown modifiable property of unit: ";