edje_cc: Keep script block during group inheritance

edje_cc: Keep script block during group inheritance

Currently, script block is removed when an edje group inherits from other group
after defining its own script block.
  group { "somegroup";
    script {
       ...
    }
    parts {
       ...
    }
    inherit: "othergroup"; // <= previous script block is removed here.
  }

If parent group doesn't have script block, it doesn't need to overwrite previous
one. This will keep script block and print warning when script block is overwritten.

Signed-Off-By: Jeeyong Um <jc9.um@samsung.com>
Change-Id: Ie49c4e5c1378658d16bf33e61586f8d5a3a7cd14