elm_win_standard: Add elm_win_standard class

elm_win_standard: Add elm_win_standard class

This commit adds the Eolian class elm_win_standard.
It is basically a derivation from elm_win that creates a default background.

The intent is to replace the legacy functions elm_win_util_standard_add and
elm_win_util_dialog_add by functions accessible via Eo API functions such as
eo_add and eo_do.

To fully replace an elm_win_util_standard_add call, use:

  eo_add(ELM_WIN_STANDARD_CLASS, NULL,
         elm_obj_win_name_set("example"),
         elm_obj_win_type_set(ELM_WIN_BASIC),
         elm_obj_win_title_set("Example"));