Themerooms: Engraving hints the location of buried treasure

Add two new themeroom functions that are called when generating
the level: pre_themerooms_generate and post_themerooms_generate,
calles before and after themerooms_generate.

Allow the buried treasure -themeroom to put down an engraving
anywhere on the level, hinting at the location of the treasure.

des.object contents function now gets the generated object passed
to it as a parameter.

dat/nhlib.lua

@@ -50,3 +50,8 @@ function monkfoodshop()
    end
    return "food shop";
 end
+
+-- pline with variable number of arguments
+function pline(fmt, ...)
+   nh.pline(string.format(fmt, table.unpack({...})));
+end